examples/mraa-gpio.c: Fixed compiler warning due to missing include

Warning fixed is atoi not being defined when building on Fedora 22

Signed-off-by: Michael Ring <mail@michael-ring.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/mraa-gpio.c

@@ -23,6 +23,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>