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/WmProperty.c

@@ -54,6 +54,7 @@ static char rcsid[] = "$TOG: WmProperty.c /main/7 1997/12/02 10:00:00 bill $"
 #include "WmColormap.h"
 #include "WmError.h"
 #include "WmResParse.h"
+#include "WmIconBox.h"