drivers: block: Mark the functions as static in skd_main.c

Mark functions skd_skmsg_state_to_str() and skd_skreq_state_to_str() as
static in skd_main.c because they are not used outside this file.

This eliminates the following warnings in skd_main.c:
drivers/block/skd_main.c:5272:13: warning: no previous prototype for ‘skd_skmsg_state_to_str’ [-Wmissing-prototypes]
drivers/block/skd_main.c:5284:13: warning: no previous prototype for ‘skd_skreq_state_to_str’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>