gpio: Include limits.h for PATH_MAX

Musl exposes this problem where PATH_MAX is used but limits.h is not
included, it works with glibc perhaps due to limits.h being indirectly
included by another system header.

Signed-off-by: Khem Raj <raj.khem@gmail.com>

src/gpio/gpio_chardev.c

@@ -12,6 +12,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <poll.h>
 #include <pthread.h>
 #include <signal.h>