Closes issue hraban/opus#9. To be honest I'm not 100% why I didn't do it
this way in the first place. Three years have passed, something was
probably fixed in CGo, or maybe I was just wrong about this not being
possible, all along. Either way, it works on my machine, so that's all
the testing you need to do, right?
pkg-config --cflags always puts you in the opus/ directory where opus.h
directly lives. No need to prefix with opus/. That just happened to work
because it was symlinked into /usr/include/opus, but we don't need to
rely on that now that we're using pkg-config.
Based on Tobias Wellnitz's comment (hraban/opus#12)