nfs_common: fix build warning in grace.c

This fix the following warning
fs/nfs_common/grace.c:66:1: warning: no previous prototype for function '__state_in_grace' [-Wmissing-prototypes]
by adding the missing static.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>