ftrace: clean up macro usage

enclose the argument in parenthesis. (especially since we cast it,
which is a high prio operation)
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/ftrace.c

@@ -165,7 +165,7 @@ static DEFINE_SPINLOCK(ftrace_hash_lock);
 #define ftrace_hash_unlock(flags) spin_lock_irqsave(&ftrace_hash_lock, flags)
 #else
 /* This is protected via the ftrace_lock with MCOUNT_RECORD. */
-#define ftrace_hash_lock(flags)   do { (void)flags; } while (0)
+#define ftrace_hash_lock(flags)   do { (void)(flags); } while (0)
 #define ftrace_hash_unlock(flags) do { } while(0)
 #endif