LU-16131 build: Do not depend on libmount during --enable-dist

LU-16131 build: Do not depend on libmount during --enable-dist

Defer the libmount requirement when using --enable-dist to
generate the lustre-src.rpm.

This allows mock and/or yum build-deps to resolve resolve
dependencies and pickup the libmount requirement without changing
the existing minimal build.

Lustre-change: https://review.whamcloud.com/48407
Lustre-commit: 819c8b169325045ae8bac9c4f38a58c75e22d099

Test-Parameters: trivial
HPE-bug-id: LUS-11091
Fixes: f21b944127 ("LU-15940 build: add a required dependency for libmount")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I20a7a097f9b651b6ea5519f79efda6c96b6f2199
Reviewed-on: https://review.whamcloud.com/48448
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>