Explicitly instantiate constructors/destructors for Kernel objects

This should speed up compile times a bit, as well as enable more liberal
use of forward declarations. (Due to SharedPtr not trying to emit the
destructor anymore.)

src/core/hle/kernel/timer.cpp

@@ -17,6 +17,9 @@ static int timer_callback_event_type = -1;
 //               us to simply use a pool index or similar.
 static Kernel::HandleTable timer_callback_handle_table;
 
+Timer::Timer() {}
+Timer::~Timer() {}
+
 ResultVal<SharedPtr<Timer>> Timer::Create(ResetType reset_type, std::string name) {
     SharedPtr<Timer> timer(new Timer);
     // TOOD(yuriks): Don't create Handle (see Thread::Create())