make dkprintf() evaluate its parameters always

Parameters of dkprintf() should be evaluated even if dkprintf() is
disabled.  Because this enables to find expression of parameter obsolete
and to avoid unnecessary compiler warnings such as "unused variable".

kernel/futex.c

@@ -76,7 +76,7 @@
 #ifdef DEBUG_PRINT_FUTEX
 #define dkprintf kprintf
 #else
-#define dkprintf(...)
+#define dkprintf(...) do { if (0) kprintf(__VA_ARGS__); } while (0)
 #endif
 
 int futex_cmpxchg_enabled;