[metacity] main: fix cast-function-type warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit a0760933bb88f283e629969efaa167705eef593a
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sun Sep 2 14:08:59 2018 +0300

    main: fix cast-function-type warning

 src/core/main.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/core/main.c b/src/core/main.c
index bccbd20f..7e6d58ec 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -324,7 +324,9 @@ sigterm_handler (int signum)
 }
 
 static gboolean
-on_sigterm (void)
+on_sigterm (GIOChannel   *source,
+            GIOCondition  condition,
+            gpointer      user_data)
 {
   meta_quit ();
   return FALSE;
@@ -372,7 +374,7 @@ main (int argc, char **argv)
 
   channel = g_io_channel_unix_new (sigterm_pipe_fds[0]);
   g_io_channel_set_flags (channel, G_IO_FLAG_NONBLOCK, NULL);
-  g_io_add_watch (channel, G_IO_IN, (GIOFunc) on_sigterm, NULL);
+  g_io_add_watch (channel, G_IO_IN, on_sigterm, NULL);
   g_io_channel_set_close_on_unref (channel, TRUE);
   g_io_channel_unref (channel);