mpegtsmux, mpegpsmux: remove GPL from choice of licenses and add SPDX license identifiers

Some people need to avoid inclusion of GPL code for their use cases and thus get easily spooked by GPL license headers. This code is actually licensed under different licenses, only one of which is GPL, and it's already possible to just upgrade from LGPL to GPL anyway so having the GPL listed explicitly as one of the choices doesn't really add anything. So remove GPL from the list and also add SPDX license identifiers while we're at it.