x11/xdm: fix build with clang 16

Clang 16 has a new error about incompatible function types, which shows
up when x11/xdm:

  greet.c:451:20: error: incompatible function pointer types assigning to 'void (*)(void)' from 'int (*)(void)' [-Wincompatible-function-pointer-types]
      __xdm_setgrent = dlfuncs->_setgrent;
		     ^ ~~~~~~~~~~~~~~~~~~

This is because it assumes setgrent(3) returns int, which it has not
done since a very long time.

Reported by:	O. Hartmann <ohartmann@walstatt.org>
PR:		272209
MFH:		2023Q2

x11/xdm/Makefile

@@ -1,5 +1,6 @@
 PORTNAME=	xdm
 PORTVERSION=	1.1.14
+PORTREVISION=	1
 CATEGORIES=	x11
 
 MAINTAINER=	x11@FreeBSD.org

x11/xdm/files/patch-include_greet.h

@@ -0,0 +1,11 @@
+--- include/greet.h.orig	2022-12-03 21:38:28 UTC
++++ include/greet.h
+@@ -45,7 +45,7 @@ from The Open Group.
+ # define GETPWNAM_ARGS /*unknown*/
+ #endif
+
+-#if defined(__FreeBSD__) || defined(__bsdi__) || defined(__osf__)
++#if defined(__bsdi__) || defined(__osf__)
+ # define SETGRENT_TYPE int
+ #else
+ # define SETGRENT_TYPE void