cpu-o3: Add M5_VAR_USED to variable

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

Change-Id: Iabe777a27d75ee8bfa7b214fff577aed3c7582c7
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/4980
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/cpu/o3/scoreboard.hh

@@ -61,7 +61,7 @@ class Scoreboard
     std::vector<bool> regScoreBoard;
 
     /** The number of actual physical registers */
-    unsigned numPhysRegs;
+    unsigned M5_CLASS_VAR_USED numPhysRegs;
 
   public:
     /** Constructs a scoreboard.