package/gdk-pixbuf: fix license

package/gdk-pixbuf: fix license

Commit a7b51ed3013c919b293deb95299e33363fb9df70 forgot to update hash of
COPYING which now contains LGPL-2.1+

Here is an extract of
https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/commit/0a8882b1a1f5234f7de2177e41f9cb584b6d61f8:

"The vast majority of GdkPixbuf is released under the terms of the GNU
Lesser General Public License, version 2.1 or later.

The following files are released under the terms of the GNU Library
General Public License, version 2 or later:
[...]
The aggregate license of the GdkPixbuf project is the Lesser GPL v2.1 or
later."

Fixes:
 - http://autobuild.buildroot.org/results/292306061216471c258a89e61b54c0d05c757321

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>