Added missing extern

src/include/gpxe/resolv.h

 extern void ignore_resolv_done ( struct resolv_interface *resolv,
 			  struct sockaddr *sa, int rc );
 extern struct resolv_interface_operations null_resolv_ops;
-struct resolv_interface null_resolv;
+extern struct resolv_interface null_resolv;
 
 extern int resolv ( struct resolv_interface *resolv, const char *name,
 		    struct sockaddr *sa );