xfs: add mising include of xfs_pnfs.h for missing declarations

The xfs_pnfs.c file is missing an include of xfs_pnfs.h to
add the prototypes of the functions it exports. Include this
file to fix the following sparse warnings:

fs/xfs/xfs_pnfs.c:27:1: warning: symbol 'xfs_break_leased_layouts' was not declared. Should it be static?
fs/xfs/xfs_pnfs.c:52:1: warning: symbol 'xfs_fs_get_uuid' was not declared. Should it be static?
fs/xfs/xfs_pnfs.c:77:1: warning: symbol 'xfs_fs_map_blocks' was not declared. Should it be static?
fs/xfs/xfs_pnfs.c:226:1: warning: symbol 'xfs_fs_commit_blocks' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>