libstrophe is dual-licensed as MIT OR GPL-3.0-only, which is also
reflected by the SPDX-License-Identifier lines in the source files.
Correct PKG_LICENSE in the Makefile accordingly.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
(cherry picked from commit ccb4632c57d456289e79e6062b3349c8779bf8ce)