_dbus_listen_systemd_sockets: fds are signed ints (-Wsign-compare)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=17289Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>

dbus/dbus-sysdeps-unix.c

@@ -1165,7 +1165,7 @@ _dbus_listen_systemd_sockets (int       **fds,
 {
 #ifdef HAVE_SYSTEMD
   int r, n;
-  unsigned fd;
+  int fd;
   int *new_fds;
 
   _DBUS_ASSERT_ERROR_IS_CLEAR (error);