squid: fix compilation with libxml (refs #19099)

Add a patch which removes a call in Libxml2Parser.cc to 'xmlSetFeature'.
This function belongs to the 'depreciated' API part and is not
available in OpenWrt builds.

According to my understanding, this call can be removed safely since
it disables the feature "substitute entities" which is disabled by default.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

net/squid/patches/020-libxml-drop-xmlSetFeature.patch

@@ -0,0 +1,10 @@
+--- a/src/esi/Libxml2Parser.cc
++++ b/src/esi/Libxml2Parser.cc
+@@ -91,7 +91,6 @@ ESILibxml2Parser::ESILibxml2Parser(ESIPa
+ 
+     /* TODO: grab the document encoding from the headers */
+     parser = xmlCreatePushParserCtxt(&sax, static_cast<void *>(this), NULL, 0, NULL);
+-    xmlSetFeature(parser, "substitute entities", 0);
+ 
+     if (entity_doc == NULL)
+         entity_doc = htmlNewDoc(NULL, NULL);