[gimp] desktop: add an <icon> tag in appdata.

[Jehan Pagès <jehanp src gnome org>]

commit e5dcd8095773e2e09734bc4e28dca12fab40c2db
Author: Jehan <jehan girinstud io>
Date:   Sat Apr 28 12:13:36 2018 +0200

    desktop: add an <icon> tag in appdata.
    
    Apparently not having it is a problem (at least for flathub), even
    though we already had this information in the desktop file.
    See: https://github.com/flathub/org.gimp.GIMP/issues/8

 desktop/org.gimp.GIMP.appdata.xml.in.in |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/desktop/org.gimp.GIMP.appdata.xml.in.in b/desktop/org.gimp.GIMP.appdata.xml.in.in
index 6a02df9..722ab0a 100644
--- a/desktop/org.gimp.GIMP.appdata.xml.in.in
+++ b/desktop/org.gimp.GIMP.appdata.xml.in.in
@@ -13,6 +13,7 @@
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-3.0+ AND LGPL-3.0+</project_license>
   <_name>GNU Image Manipulation Program</_name>
+  <icon type="stock">gimp</icon>
   <_summary>Create images and edit photographs</_summary>
   <description>
     <_p>