mem-ruby: Added M5_CLASS_VAR_USED to m_id in OutputUnit

mem-ruby: Added M5_CLASS_VAR_USED to m_id in OutputUnit

Clang 9 throws an error that 'm_id' is unused (encountered when
compiling X86.fast). M5_CLASS_VAR_USED has been added to avoid this
error.

Change-Id: I722edd1429a074ff484b5ebbdc431af0089561b5
Issue-on: https://gem5.atlassian.net/browse/GEM5-560
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29304
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>