desktop: add a <developer_name> tag.

It just says "The GIMP team" so it's kind of redundant/useless, but I
noticed that Flathub would just display an empty "Developer" section
because the tag is absent. Well at least it emphasizes the
community-developed side of GIMP.

desktop/org.gimp.GIMP.appdata.xml.in.in

@@ -12,6 +12,7 @@
   -->
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-3.0+ AND LGPL-3.0+</project_license>
+  <developer_name>The GIMP team</developer_name>
   <_name>GNU Image Manipulation Program</_name>
   <_summary>Create images and edit photographs</_summary>
   <description>