syscall emulation: fix fast build issue

Surprisingly gcc will complain about unused variables even
inside an 'if (false)' block.

I thought I had tested this previously, but apparently not.

src/sim/syscall_emul.cc

@@ -57,7 +57,7 @@ SyscallDesc::doSyscall(int callnum, LiveProcess *process, ThreadContext *tc)
 {
     if (DTRACE(SyscallVerbose)) {
         int index = 0;
-        IntReg arg[4];
+        IntReg arg[4] M5_VAR_USED;
 
         // we can't just put the calls to getSyscallArg() in the
         // DPRINTF arg list, because C++ doesn't guarantee their order