dtwm: Cleanup some implicit definitions

Mostly this is adding appropriate #includes and declarations,
but for WmImage.c we also change from using the proper name
for XmeGetMask, rather than the identical but renamed
version _DtGetMask which is not exported in any header.

cde/programs/dtwm/WmWinState.c

@@ -42,6 +42,7 @@ static char rcsid[] = "$XConsortium: WmWinState.c /main/6 1996/06/20 09:39:39 rs
 #include "WmGlobal.h"
 #include "WmICCC.h"
 #include "WmProtocol.h"
+#include "WmIPC.h"
 
 
 /*