[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/open.h

@@ -9,10 +9,10 @@
 
 #include <stdarg.h>
 #include <gpxe/tables.h>
+#include <gpxe/socket.h>
 
 struct xfer_interface;
 struct uri;
-struct sockaddr;
 
 /** Location types */
 enum {