package/libgdiplus: bump to version 5.6

As part of this bump, we backport two upstream patches that fix the
license text to really reflect the license of the project. The second
patch was prompted by a bug report made by Arnout Vandecappelle
(https://github.com/mono/libgdiplus/issues/375

), following a
discussion on the Buildroot mailing list. The first patch is needed as
a dependency of this first patch. Since both patches are upstream,
they can be dropped during the next version bump.

So now, the license text is the one of the MIT license, which matches
the header comments in all source files, making the comment about the
<pkg>_LICENSE variable in libgdiplus.mk irrelevant. The hash of the
license file is updated as well.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: update licensing aspects.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>