rules: quiet a gcc warning

src/xkbcomp/rules.c:620:36: error: 'idx' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Can't happen but no harm done.

Signed-off-by: Ran Benita <ran234@gmail.com>

src/xkbcomp/rules.c

@@ -555,6 +555,7 @@ static int
 extract_layout_index(const char *s, size_t max_len, xkb_layout_index_t *out)
 {
     /* This function is pretty stupid, but works for now. */
+    *out = XKB_LAYOUT_INVALID;
     if (max_len < 3)
         return -1;
     if (s[0] != '[' || !isdigit(s[1]) || s[2] != ']')