LU-17843 build: mount.lustre_tgt as symlink

LU-17843 build: mount.lustre_tgt as symlink

mount.lustre_tgt was always intended to be a symlink to the
main mount.lustre, but instead it's an identical build item.

This results in this rpmbuild warning (an error on some systems):
    Duplicate build-ids .../mount.lustre and .../mount.lustre_tgt

This patch resolves this by making it a proper symlink as was
originally intended.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: Id5f90df94fcc5c73a93fd6c1311b09af429c1440
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55079
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>