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/timer.c

@@ -38,7 +38,7 @@
 #ifdef DEBUG_PRINT_TIMER
 #define dkprintf kprintf
 #else
-#define dkprintf(...)
+#define dkprintf(...) do { if (0) kprintf(__VA_ARGS__); } while (0)
 #endif
 
 #define LOOP_TIMEOUT 10