Add more missing config.h includes

config.h includes were missing in a few files, including input.c, the
lack of which caused the X11 backend to segfault instantly due to not
having an xkbcommon context.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/src/data-device.c b/src/data-device.c
index 4bf6256..25587b2 100644
--- a/src/data-device.c
+++ b/src/data-device.c
@@ -20,6 +20,8 @@
  * OF THIS SOFTWARE.
  */
 
+#include "config.h"
+
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>