block: Declare local symbols static

Avoid that building with W=1 causes the compiler to complain that
a declaration for bounce_bio_set and bounce_bio_split is missing.

References: commit a8821f3f ("block: Improvements to bounce-buffer handling")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Neil Brown <neilb@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bounce.c

@@ -26,7 +26,7 @@
 #define POOL_SIZE	64
 #define ISA_POOL_SIZE	16
 
-struct bio_set *bounce_bio_set, *bounce_bio_split;
+static struct bio_set *bounce_bio_set, *bounce_bio_split;
 static mempool_t *page_pool, *isa_page_pool;
 
 #if defined(CONFIG_HIGHMEM) || defined(CONFIG_NEED_BOUNCE_POOL)