mem-cache: Fix unused variable warning in FALRU:invalidate()

Change-Id: I3b902045433ca56b3e62c251158e784b5fa9e4d7
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13600
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/mem/cache/tags/fa_lru.cc

@@ -124,7 +124,8 @@ void
 FALRU::invalidate(CacheBlk *blk)
 {
     // Erase block entry reference in the hash table
-    auto num_erased = tagHash.erase(std::make_pair(blk->tag, blk->isSecure()));
+    auto num_erased M5_VAR_USED =
+        tagHash.erase(std::make_pair(blk->tag, blk->isSecure()));
 
     // Sanity check; only one block reference should be erased
     assert(num_erased == 1);