Add a thread safe variant of invalidating the icache

This is used by the next commit.

Source/Core/Core/PowerPC/PowerPC.h

@@ -138,6 +138,7 @@ extern PPCDebugInterface debug_interface;
 void Init(int cpu_core);
 void Shutdown();
 void DoState(PointerWrap& p);
+void ScheduleInvalidateCacheThreadSafe(u32 address);
 
 CoreMode GetMode();
 // [NOT THREADSAFE] CPU Thread or CPU::PauseAndLock or CORE_UNINITIALIZED