network: Undefine existing gai_strerror definitions

This avoids warnings if there already exists a definition.

This is the case on windows, where the getaddrinfo isn't available
and linked to (and we use our fallbacks instead, which actually
try to use the proper getaddrinfo version if found at runtime),
but gai_strerror still exists as a define.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/network.h

@@ -181,6 +181,7 @@ const char *ff_gai_strerror(int ecode);
 #define getaddrinfo ff_getaddrinfo
 #define freeaddrinfo ff_freeaddrinfo
 #define getnameinfo ff_getnameinfo
+#undef gai_strerror
 #define gai_strerror ff_gai_strerror
 #endif