[rhythmbox] bump gtk+ requirement to 3.20

[Jonathan Matthew <jmatthew src gnome org>]

commit a6cb89827a98cac959c024e6248b578f51add708
Author: Jonathan Matthew <jonathan d14n org>
Date:   Sat Nov 4 17:31:40 2017 +1000

    bump gtk+ requirement to 3.20
    
    See https://bugzilla.gnome.org/show_bug.cgi?id=789839

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 746dfb4..5fd48b3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -42,7 +42,7 @@ m4_ifdef([LT_OUTPUT], [LT_OUTPUT])
 AC_C_BIGENDIAN
 AC_CHECK_SIZEOF(long)
 
-GTK_REQS=3.16.0
+GTK_REQS=3.20.0
 
 GST_REQS=1.4.0
 GDK_PIXBUF_REQS=2.18.0