stats: Add M5_VAR_USED to variable
stats: Add M5_VAR_USED to variable

Fixes compile error for gem5.fast due to unused variable.

Change-Id: Ib1664b100e95a741174610f8cdbdebaff9635f66
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20759
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5