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>