[media] pwc: fix sparse warning

drivers/media/usb/pwc/pwc-v4l.c:55:12: warning: symbol 'pwc_auto_whitebal_qmenu' was not declared. Should it be static?

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>