x11: fix comparison of integer expressions of different signedness

src/x11/keymap.c:980:26: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
  980 |     for (size_t i = 0; i < length; i++) {
      |                          ^

Signed-off-by: Ran Benita <ran@unusedvar.com>

src/x11/keymap.c

@@ -977,7 +977,7 @@ get_group_names(struct xkb_keymap *keymap, struct x11_atom_interner *interner,
     keymap->num_group_names = msb_pos(reply->groupNames);
     ALLOC_OR_FAIL(keymap->group_names, keymap->num_group_names);
 
-    for (size_t i = 0; i < length; i++) {
+    for (int i = 0; i < length; i++) {
         x11_atom_interner_adopt_atom(interner, iter[i],
                                      &keymap->group_names[i]);
     }