[gnome-applets] Bump LIBPANEL_REQUIRED to 3.18.0

[Alberts Muktupāvels <muktupavels src gnome org>]

commit fd7f49be35cc7832627fd08af3616e7a70b7db07
Author: Dmitry Shachnev <mitya57 gmail com>
Date:   Mon Sep 28 20:16:26 2015 +0300

    Bump LIBPANEL_REQUIRED to 3.18.0
    
    Window-picker-applet uses panel_applet_get_gtk_orientation,
    which was added to libpanel-applet in 3.18.0 release.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 3e62541..979be20 100644
--- a/configure.ac
+++ b/configure.ac
@@ -55,7 +55,7 @@ dnl ***************************************************************************
 GTK_REQUIRED=3.15.2
 GLIB_REQUIRED=2.44.0
 GIO_REQUIRED=2.26.0
-LIBPANEL_REQUIRED=3.16.1
+LIBPANEL_REQUIRED=3.18.0
 LIBGTOP_REQUIRED=2.11.92
 LIBXKLAVIER_REQUIRED=4.0
 LIBWNCK_REQUIRED=2.91.0