[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/socket.h

@@ -94,6 +94,6 @@ struct sockaddr {
 	 * family.
 	 */
 	char pad[ SA_LEN - sizeof ( sa_family_t ) ];
-};
+} __attribute__ (( may_alias ));
 
 #endif /* _GPXE_SOCKET_H */