Fix build for CONFIG_GENERIC_GPIO=n, which causes this warning:

include/linux/gpio.h:93: warning: 'struct device' declared inside parameter list
include/linux/gpio.h:93: warning: its scope is only this definition or declaration, which is probably not what you want
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jani Nikula <ext-jani.1.nikula@nokia.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Daniel Silverstone <dsilvers@simtec.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/gpio.h

@@ -12,6 +12,8 @@
 #include <linux/types.h>
 #include <linux/errno.h>
 
+struct device;
+
 /*
  * Some platforms don't support the GPIO programming interface.
  *