rtc: ds1685: use EXPORT_SYMBOL_GPL for ds1685_rtc_poweroff

ds1685_rtc_poweroff is only used externally via symbol_get, which was
only ever intended for very internal symbols like this one.  Use
EXPORT_SYMBOL_GPL for it so that symbol_get can enforce only being used
on EXPORT_SYMBOL_GPL symbols.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Joshua Kinard <kumba@gentoo.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>