Fix warning when compiling socketmodule.c with VS2010

VS2010 defineds the old errno constants in addition to the WSA* ones.

Modules/socketmodule.c

@@ -390,6 +390,7 @@ dup_socket(SOCKET handle)
 #endif
 
 #ifdef MS_WIN32
+#undef EAFNOSUPPORT
 #define EAFNOSUPPORT WSAEAFNOSUPPORT
 #define snprintf _snprintf
 #endif