Xming: Fix a couple of warnings

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>

hw/xwin/winmultiwindowwm.c

@@ -798,6 +798,7 @@ winMultiWindowWMProc (void *pArg)
 #if CYGMULTIWINDOW_DEBUG
   ErrorF("-winMultiWindowWMProc ()\n");
 #endif
+  return NULL;
 }
 
 
@@ -1058,7 +1059,7 @@ winMultiWindowXMsgProc (void *pArg)
 
   XCloseDisplay (pProcArg->pDisplay);
   pthread_exit (NULL);
- 
+  return NULL;
 }