fix errors

kernel/syscall/handler.cc

@@ -41,6 +41,7 @@ namespace Syscall {
 extern "C" size_t syscall_handler(size_t sysnum, size_t p1, size_t p2,
 								  size_t p3, size_t p4, size_t p5,
 								  InterruptContext *user) {
+    (void) user;
     switch ((Syscall::ID)sysnum) {
         case Syscall::ID::TEST:
             return Syscall::Skeleton::test(Guard::enter().vault(), p1, p2, p3, p4, p5);