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.

unix/gtkdlg.c

@@ -3960,7 +3960,7 @@ void showeventlog(void *estuff, void *parentwin)
     c->listbox.percentages[2] = 65;
 
     es->window = window = our_dialog_new();
-    title = dupcat(appname, " Event Log", NULL);
+    title = dupcat(appname, " Event Log", (const char *)NULL);
     gtk_window_set_title(GTK_WINDOW(window), title);
     sfree(title);
     w0 = layout_ctrls(&es->dp, &es->scs, s0, GTK_WINDOW(window));

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,