dtpad: Fix 64-bit crash on file open/save

Fix SIGSEGV because of implicit declaration
of _XmStringUngenerate.

The error message reported to the user was:

TT_ERR_PROCID The process id passed is not valid.

cde/programs/dtpad/fileCB.c

@@ -59,6 +59,8 @@
 #include <Dt/DtpadM.h>
 #include <Dt/Action.h>
 
+#include <Xm/XmPrivate.h> /* _XmStringUngenerate */
+
 #include "dtpad.h"
 
 extern int numActivePads;  /* declared in main.c */