selftest/gpio: Remove duplicate header

Remove duplicate header which are included twice.
Signed-off-by: Sabyasachi Gupta <sabyasachi.linux@gmail.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/gpio/gpio-mockup-chardev.c

@@ -12,7 +12,6 @@
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <stdio.h>
 #include <errno.h>
 #include <string.h>
 #include <fcntl.h>