LU-15940 build: add a required dependency for libmount

LU-15940 build: add a required dependency for libmount

The Lustre client utilities (mount/umount) have
an optional dependency on libmount to update utab.
However, libmount has been introduced in util-linux
since 2.18 in 2010. There is no need to make the
dependency as optional.

Lustre-change: https://review.whamcloud.com/47619
Lustre-commit: f21b9441275bd0d8b0d56cf8441aa67bede37ddc

Test-Parameters: trivial clientdistro=ubuntu2004
Test-Parameters: trivial clientdistro=el8.5

Change-Id: I4b965a5ce6cb6fc5d2061a53c44ef9b709ebab49
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47759
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>