DtSvc/dtwm: Fix some implicit declarations of functions by adding appropriate

Patch from Frederic Koehler <f.koehler427@gmail.com>:

These implicit definitions cause segfaults on x64 because
the implicit return type is a 32-bit signed int, rather than a pointer
type.

cde/programs/dtwm/Button.c

@@ -38,6 +38,7 @@ static char SCCSID[] = "OSF/Motif: @(#)Button.c	1.19 95/05/01";
 #include <X11/cursorfont.h>
 #include "ButtonP.h"
 #include <Xm/ManagerP.h>
+#include <Xm/DrawP.h>
 #include <Dt/Control.h>
 #include <Dt/MacrosP.h>
 #include <Dt/DtStrDefs.h>