stubdom: fix error assignment in grub:load_module

[ 1333s] mini-os.c: In function 'load_module':
[ 1333s] mini-os.c:244: warning: statement with no effect

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

stubdom/grub/mini-os.c

@@ -241,7 +241,7 @@ load_module (char *module, char *arg)
 
     if ((void*) (multiboot_next_module_header+1) - module_image > PAGE_SIZE) {
         /* Too many modules */
-        ERR_WONT_FIT;
+        errnum = ERR_WONT_FIT;
         return 0;
     }