Add wxUSE_THREADS checks around wxMSW functions dealing with threads.

This removes the code unneeded in wxUSE_THREADS==0 build and also fixes
compilation of wxGUIAppTraits::WaitForThread() which didn't compile any more
with wxUSE_THREADS==0 since r67185.

Closes #13050.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67288 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/palmos/apptbase.h

@@ -31,12 +31,14 @@ public:
     virtual void AfterChildWaitLoop(void *data) = 0;
 
 
+#if wxUSE_THREADS
     // wxThread helpers
     // ----------------
 
     // process a message while waiting for a(nother) thread, should return
     // false if and only if we have to exit the application
     virtual bool DoMessageFromThreadWait() = 0;
+#endif // wxUSE_THREADS
 };
 
 #endif // _WX_PALMOS_APPTBASE_H_