RTL-SDR for GNU Radio v3.7

Since we’re moving now towards the release of version 3.7 and the
master branch has been updated to track that progress, all libraries
that build off of and talk to GNU Radio must be updated appropriately.

I’ve already started to do this with gr-osmosdr. Right now, I’ve
updated their interface to the RTL-SDR (tested) and HackRF (untested)
to compile against the new master branch of GNU Radio. You can use my
repo here as a remote with your current gr-osmosdr to get the gr-next
branch to build.