block: make blk_try_req_merge() static

blk_try_req_merge() is only used in block/blk-merge.c, so make it
static.

This addresses a gcc warning when -Wmissing-prototypes is enabled.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>