tablet-shell: Fix compiling error for using stderr.

Not including the stdio.h makes this compiling error.

src/tablet-shell.c

@@ -23,6 +23,7 @@
 #include <sys/wait.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <linux/input.h>