Move compat/ and bstr/ directory contents somewhere else

bstr.c doesn't really deserve its own directory, and compat had just
a few files, most of which may as well be in osdep. There isn't really
any justification for these extra directories, so get rid of them.

The compat/libav.h was empty - just delete it. We changed our approach
to API compatibility, and will likely not need it anymore.

audio/decode/ad_lavc.c

@@ -37,8 +37,6 @@
 #include "ad.h"
 #include "audio/fmt-conversion.h"
 
-#include "compat/libav.h"
-
 struct priv {
     AVCodecContext *avctx;
     AVFrame *avframe;