[sysprof] build: drop back to 3.21.5 so we can build on nightly

[Christian Hergert <chergert src gnome org>]

commit fa2543538820ae1d955cc91150ff5442c75c36ba
Author: Christian Hergert <chergert redhat com>
Date:   Thu Sep 15 13:39:14 2016 -0700

    build: drop back to 3.21.5 so we can build on nightly
    
    Nightly hasn't been updated to 3.21.6 yet.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 5b580ed..cb96c7a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -112,7 +112,7 @@ dnl ***********************************************************************
 dnl Check for required packages
 dnl ***********************************************************************
 m4_define([glib_required_version], [2.44.0])
-m4_define([gtk_required_version], [3.21.6])
+m4_define([gtk_required_version], [3.21.5])
 m4_define([systemd_required_version], [222])
 
 PKG_CHECK_MODULES(GIO,