[gnome-builder] build: bump gtk+ requirement

[Christian Hergert <chergert src gnome org>]

commit ae284beccc867115c8e351fefcc05974a12e0f65
Author: Christian Hergert <chergert redhat com>
Date:   Tue Sep 13 14:58:28 2016 -0700

    build: bump gtk+ requirement
    
    We require natural sizing fixes which were part of 3.21.6

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index be06d18..d240609 100644
--- a/configure.ac
+++ b/configure.ac
@@ -182,7 +182,7 @@ AC_SUBST(BUGREPORT_URL)
 dnl ***********************************************************************
 dnl Check for Required Packages
 dnl ***********************************************************************
-m4_define([gtk_required_version], [3.21.5])
+m4_define([gtk_required_version], [3.21.6])
 m4_define([glib_required_version], [2.49.0])
 m4_define([gtksourceview_required_version], [3.21.2])
 m4_define([gobject_introspection_version], [1.48.0])