LU-16661 build: use "Recommends: perl" for lustre-iokit

LU-16661 build: use "Recommends: perl" for lustre-iokit

In lustre-iokit, the "plot" commands all use perl, but
the actual "*-survey" scripts are written in bash, so
the "Requires: perl" in lustre.spec.in for lustre-iokit
could be downgraded to "Recommends: perl" for RHEL 8+
(RHEL 7 does not handle "Recommends:").

Test-Parameters: trivial testlist=obdfilter-survey

Change-Id: I55f3c57e73ac91cedce745dc4f424c3542978cd4
Fixes: 800a9ec58f78 ("LU-16661 build: improve lustre.spec.in Requires")
Signed-off-by: Jian Yu <yujian@whamcloud.com>