drivers: block: Mark function as static in drbd_actlog.c

Mark the function drbd_al_begin_io_prepare() as static in
drbd/drbd_actlog.c because it is not used outside this file.

This eliminates the following warnings in drbd/drbd_actlog.c:
drivers/block/drbd/drbd_actlog.c:277:6: warning: no previous prototype for ‘drbd_al_begin_io_prepare’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>