lavc: rename h265 to hevc, add AV_CODEC_ID_H265 with identical value for backward compatibility

Somehow 2 different identifiers have come into existence which makes supporting
both until the next major version bump required
I did not investigate how exactly that did happen.

wm4, j-b and smarter prefer hevc.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>