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.

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/47619
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>