Xext: fix missing include of <errno.h>

It's much cleaner to always include directly what one needs,
instead of relying on very indirect including.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1435>

Xext/xselinux_hooks.c

@@ -26,6 +26,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include <dix-config.h>
 #endif
 
+#include <errno.h>
 #include <sys/socket.h>
 #include <stdio.h>
 #include <stdarg.h>