Slightly speed up booting when there are symbols to load.

Also avoid an obscure hang I managed to trigger..

Windows/WindowsHost.cpp

@@ -237,7 +237,7 @@ void WindowsHost::PollControllers() {
 
 void WindowsHost::BootDone() {
 	g_symbolMap->SortSymbols();
-	SendMessage(mainWindow_, WM_USER + 1, 0, 0);
+	PostMessage(mainWindow_, WM_USER + 1, 0, 0);
 
 	SetDebugMode(!g_Config.bAutoRun);
 	Core_EnableStepping(!g_Config.bAutoRun);