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.

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/48407
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>