staging: dgap: fix sparse warning about dgap_poll_lock

This patch fixes a sparse warning:

warning: symbol 'dgap_poll_lock' was not declared. Should it be static?
Signed-off-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgap/dgap.c

@@ -226,7 +226,7 @@ static uint dgap_count = 500;
 /*
  * Poller stuff
  */
-DEFINE_SPINLOCK(dgap_poll_lock);	/* Poll scheduling lock */
+static DEFINE_SPINLOCK(dgap_poll_lock);	/* Poll scheduling lock */
 static ulong dgap_poll_time;		/* Time of next poll */
 static uint dgap_poll_stop;		/* Used to tell poller to stop */
 static struct timer_list dgap_poll_timer;