Fix build with gcc-14

Upstream: https://sources.debian.org/src/xvkbd/4.1-3/debian/patches/gcc-14

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
---
 xvkbd.c |    1 +
 1 file changed, 1 insertion(+)

--- a/xvkbd.c
+++ b/xvkbd.c
@@ -15,6 +15,7 @@
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  * See the GNU General Public License for more details.
  */
+#define _CONST_X_STRING
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>