Removes the special -I flag specified in the avcodec/hevc/ subdirectory.
This makes code copy-pastable to other parts of the ffmpeg codebase, as
well as simplifying the build script.
It also reduces ambiguity, since there are many instances of same-named
header files existing in both libavformat/ and libavcodec/
subdirectories.
Follows: b29bdd3715
