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 <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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