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/WmICCC.h

@@ -31,6 +31,8 @@
 /*
  * (c) Copyright 1987, 1988, 1989, 1990 HEWLETT-PACKARD COMPANY */
 
+#ifndef _Dt_WmICCC_h_
+#define _Dt_WmICCC_h_
 /*
  * Included Files:
  */
@@ -143,3 +145,4 @@ typedef struct _PropWMState
 
 /* PropWMState state field value: */
 #define WithdrawnSTATE		0
+#endif /* _Dt_WmICCC_h_ */