LU-14435 doc: include lfs-flushctx manpage inside packages

lfs manpage redirects to lfs-flushctx(1), so it has to be
included in the Lustre packages.

Test-Parameters: trivial
Fixes: c246a9ba04 ("LU-14263 gss: unlink revoked key")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I5c55f8a74eb6dac20fa85b6ea0663ad701341006
Reviewed-on: https://review.whamcloud.com/41676
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>