backend: drop unconditional and unused <libinput.h>

After 70fb21c35b made libinput optional the include prevents
building without libinput package installed.

backend/backend.c:4:10: fatal error: 'libinput.h' file not found
 #include <libinput.h>
          ^~~~~~~~~~~~

backend/backend.c

@@ -1,7 +1,6 @@
 #define _POSIX_C_SOURCE 200809L
 #include <assert.h>
 #include <errno.h>
-#include <libinput.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>