gpio: Fix ARM versatile-express build failure

A missing mach/gpio.h prevents building gpiolib on versatile express.

  CC      drivers/gpio/gpiolib.o
In file included from /.../linux/include/linux/gpio.h:18:0,
                 from /.../linux/drivers/gpio/gpiolib.c:10:
/.../linux/arch/arm/include/asm/gpio.h:5:23: fatal error: mach/gpio.h: No such file or directory
compilation terminated.
make[3]: *** [drivers/gpio/gpiolib.o] Error 1
make[2]: *** [drivers/gpio] Error 2
make[1]: *** [drivers] Error 2
make: *** [sub-make] Error 2

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/arm/mach-vexpress/include/mach/gpio.h

@@ -0,0 +1 @@
+/* empty */