libstrophe: Specify correct PKG_LICENSE
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)
Edited by Andreas Gnau