[build] Avoid strict-aliasing warnings when building with gcc 4.4

Conventional usage of the various struct sockaddr_xxx types involves
liberal use of casting, which tends to trigger strict-aliasing
warnings from gcc.  Avoid these now and in future by marking all the
relevant types with __attribute__((may_alias)).

src/include/gpxe/resolv.h

@@ -10,8 +10,8 @@
 #include <gpxe/refcnt.h>
 #include <gpxe/interface.h>
 #include <gpxe/tables.h>
+#include <gpxe/socket.h>
 
-struct sockaddr;
 struct resolv_interface;
 
 /** Name resolution interface operations */