cpu: fixed unused variable on fast binary

cpu: fixed unused variable on fast binary

When gem5.fast is compiled, an error on a variable
used only for debug purposes is raised:

build/X86/cpu/o3/mem_dep_unit_impl.hh:262:19: error: unused variable 'producing_store' [-Werror=unused-variable]
         for (auto producing_store : producing_stores)

This patch remove the variable when *.fast is used.

Change-Id: Ib77c26073db39644e3525bc16edcb7d3bc871d76
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29252
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>