staging: bcm2835: don't mark 'bcm2835_v4l2_debug' as static

This one unfortunately slipped through my own build testing, my patch
caused a new build error:

bcm2835-camera.c:53:12: error: static declaration of 'bcm2835_v4l2_debug' follows non-static declaration

We want the symbol to be global as it is indeed used in more than one
file and declared 'extern' in a header.

Fixes: 757b9bd0 ("staging: bcm2835: mark all symbols as 'static'")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>