Updated gr-osmosdr for GNU Radio 3.7

Not sure if anyone else had worked on this, but I couldn't find anything. I updated gr-osmosdr to work with GNU Radio 3.7 (currently the next branch). You can find it here: https://github.com/trondeau/gr-osmosdr I didn't redo the code to be in the 3.7 API style, just made sure that it could build off gnuradio and run properly. Tom