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/WmIPC.c

@@ -38,7 +38,9 @@ static char rcsid[] = ""
  * Included Files:
  */
 #include "WmGlobal.h"
+#include "WmProtocol.h"
 #include <Xm/XmAll.h>
+#include <Dt/Dt.h>
 #include <Dt/DtP.h>
 #include <Dt/Action.h>
 #include <Dt/WsmM.h>
@@ -51,6 +53,8 @@ static char rcsid[] = ""
 #include "WmFunction.h"
 #include "WmWrkspace.h"
 #include "WmIPC.h"
+#include "WmXSMP.h"
+#include "WmPanelP.h"
 #include "DataBaseLoad.h"