Build dtls_bench without the server or the client side

examples/benchmark/dtls_bench.c calls wolfSSL_accept and wolfSSL_connect
unconditionally, so a build that compiles either side out fails to link:

  Undefined symbols: "_wolfSSL_accept", referenced from: _main in dtls_bench.o

That is the os-check dtls13-client-minimal entry, which passes
CPPFLAGS=-DNO_WOLFSSL_SERVER; os-check.yml does not pass --disable-examples,
so the benchmark is always built.

Compile each DTLS leg out with its own side, the way examples/server/server.c
does, and report the missing side from main(). pick_method() and set_mtu()
serve only those two legs and go with them, and the send sink is used by the
client leg alone, so none of them are left unused. The plain UDP legs need
neither side and keep working.

examples/benchmark/dtls_bench.c

@@ -124,6 +124,7 @@ static double now_sec(void)
     return (double)ts.tv_sec + (double)ts.tv_nsec / 1e9;
 }
 
+#ifndef NO_WOLFSSL_CLIENT
 /* Post-handshake send sink. Pretends every byte was transmitted but does
  * nothing; the kernel's UDP/IP path never runs. Used via -z to measure the
  * pure wolfSSL encrypt-and-frame ceiling, free of any I/O cost. */
@@ -132,6 +133,7 @@ static int dtls_bench_sink_send(WOLFSSL* ssl, char* buf, int sz, void* ctx)
     (void)ssl; (void)buf; (void)ctx;
     return sz;
 }
+#endif /* !NO_WOLFSSL_CLIENT */
 
 static void print_stats(const char* dir, long long bytes, double sec)
 {
@@ -338,6 +340,9 @@ static int bind_to_iface(int fd, const char* ifname)
 #endif
 }
 
+/* Only the DTLS legs below need these; the plain-UDP baseline does not. */
+#if !defined(NO_WOLFSSL_SERVER) || !defined(NO_WOLFSSL_CLIENT)
+
 static WOLFSSL_METHOD* pick_method(int version, int isServer)
 {
     if (version == 13) {
@@ -404,6 +409,8 @@ static int set_mtu(WOLFSSL* ssl, int mtu)
 #endif
 }
 
+#endif /* !NO_WOLFSSL_SERVER || !NO_WOLFSSL_CLIENT */
+
 /* ----- Plain-UDP baseline (-n) ----- */
 
 static int udp_server(const cfg_t* c)
@@ -523,6 +530,8 @@ out:
 
 /* ----- DTLS server ----- */
 
+#ifndef NO_WOLFSSL_SERVER
+
 static int dtls_server(const cfg_t* c)
 {
     int                ret      = 1;
@@ -658,8 +667,12 @@ cleanup:
     return ret;
 }
 
+#endif /* !NO_WOLFSSL_SERVER */
+
 /* ----- DTLS client ----- */
 
+#ifndef NO_WOLFSSL_CLIENT
+
 static int dtls_client(const cfg_t* c)
 {
     int                ret      = 1;
@@ -797,6 +810,8 @@ cleanup:
     return ret;
 }
 
+#endif /* !NO_WOLFSSL_CLIENT */
+
 int main(int argc, char** argv)
 {
     cfg_t c;
@@ -807,7 +822,20 @@ int main(int argc, char** argv)
     if (c.plainUdp) {
         return c.isServer ? udp_server(&c) : udp_client(&c);
     }
-    return c.isServer ? dtls_server(&c) : dtls_client(&c);
+    if (c.isServer) {
+#ifndef NO_WOLFSSL_SERVER
+        return dtls_server(&c);
+#else
+        fprintf(stderr, "DTLS server not compiled in!\n");
+        return 1;
+#endif
+    }
+#ifndef NO_WOLFSSL_CLIENT
+    return dtls_client(&c);
+#else
+    fprintf(stderr, "DTLS client not compiled in!\n");
+    return 1;
+#endif
 }
 
 #else /* DTLS_BENCH_ENABLED */