Add missing casts in dupcat().

Ahem. I _spotted_ this in code review, and forgot to make the change
before pushing!

Because it's legitimate for a C implementation to define 'NULL' so
that it expands to just 0, it follows that if you use NULL in a
variadic argument list where the callee will expect to extract a
pointer, you run the risk of putting an int-sized rather than
pointer-sized argument on the list and causing the consumer to get out
of sync. So you have to add an explicit cast.

windows/windlg.c

@@ -779,7 +779,7 @@ void logevent(void *frontend, const char *string)
 
     if (*location)
         sfree(*location);
-    *location = dupcat(timebuf, string, NULL);
+    *location = dupcat(timebuf, string, (const char *)NULL);
     if (logbox) {
 	int count;
 	SendDlgItemMessage(logbox, IDN_LIST, LB_ADDSTRING,