[PATCH] add xtrx support

Dear Maintainers, The attached patch adds XTRX SDR support to Gr-Osmosdr. It mostly contains the out-of tree patches added in the libxtrx branch here: https://github.com/xtrx-sdr/gr-osmosdr , also the patches upstreamed to be compatible with Gnu-Radio 3.8 and the latest master branch of gr-osmosdr. It compiles and works fine with Gnu-Radio 3.8 and Gr-Osmosdr master (tested on Ubuntu 20.04.01 LTS 64bit). Hopefully I will have time to test it against other Osmocom projects in the near future. Please let me know if the patch needs any modifications or further work. Regards, Csaba