[gnome-screenshot] Bump GTK+ requirement to 3.12.0

[Andre Klapper <aklapper src gnome org>]

commit 9b32d5a2167f1e6ce9c0535d13cc2ee554df0f62
Author: Andre Klapper <a9016009 gmx de>
Date:   Thu Jan 3 20:36:57 2019 +0100

    Bump GTK+ requirement to 3.12.0
    
    Required for gtk_widget_set_margin_start(). Fixes bug #761946.

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 1573838..7d0b1e8 100644
--- a/meson.build
+++ b/meson.build
@@ -23,7 +23,7 @@ gnome_screenshot_servicesdir = join_paths(gnome_screenshot_datadir, 'dbus-1', 's
 gnome_screenshot_schemadir = join_paths(gnome_screenshot_datadir, 'glib-2.0', 'schemas')
 
 glib_req_version = '>= 2.35.1'
-gtk_req_version = '>= 3.0.0'
+gtk_req_version = '>= 3.12.0'
 
 mathlib_dep = cc.find_library('m', required: false)
 x11_dep = [ dependency('x11'), dependency('xext') ]