Use the new wxSysErrorMsgStr() instead of wxSysErrorMsg()

src/msw/thread.cpp

@@ -1371,7 +1371,7 @@ void WXDLLIMPEXP_BASE wxWakeUpMainThread()
             wxS("Failed to wake up main thread: PostThreadMessage(WM_NULL) ")
             wxS("failed with error 0x%08lx (%s)."),
             ec,
-            wxSysErrorMsg(ec)
+            wxSysErrorMsgStr(ec)
         );
     }
 }