btrfs: make btrfs_should_periodic_reclaim() static

btrfs_should_periodic_reclaim() is not used outside of space-info.c so
make it static and remove the prototype from space-info.h.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>