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/WmOL.c

@@ -37,6 +37,8 @@ static char rcsid[] = "$XConsortium: WmOL.c /main/4 1995/11/01 11:45:08 rswiston
  */
 #include "WmGlobal.h"
 #include "WmOL.h"
+#include "WmProperty.h"
+#include <Xm/AtomMgr.h>
 
 #define ValidPropertyList(pcd) ((pcd)->paInitialProperties != NULL)