watchdog: pnx4008: make global wdt_clk static

Silences sparse warning:

  drivers/watchdog/pnx4008_wdt.c:83:25:
    warning: symbol 'wdt_clk' was not declared. Should it be static?
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/pnx4008_wdt.c

@@ -80,7 +80,7 @@ static unsigned int heartbeat = DEFAULT_HEARTBEAT;
 
 static DEFINE_SPINLOCK(io_lock);
 static void __iomem	*wdt_base;
-struct clk		*wdt_clk;
+static struct clk	*wdt_clk;
 
 static int pnx4008_wdt_start(struct watchdog_device *wdd)
 {