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]