Avoid that building with W=1 causes gcc to report warnings about symbols
that have not been declared.

Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: NBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: NHannes Reinicke <hare@suse.de>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NEwan Milne <emilne@redhat.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>