Install the apt-file by using

apt install apt-file

Search for avcodec.h by using the following command

apt-file search avcodec.h

It will the show all the availability of the avcodec.h

ffmpeg-doc: /usr/share/doc/ffmpeg/manual/libavcodec.html
libaubio-dev: /usr/include/aubio/io/source_avcodec.h
libavcodec-dev: /usr/include/x86_64-linux-gnu/libavcodec/avcodec.h
libmyth-dev: /usr/include/mythtv/libavcodec/avcodec.h
libvlccore-dev: /usr/include/vlc/plugins/vlc_avcodec.h
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/cs/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/de/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/en/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/fr/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/hu/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/it/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/pl/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/ru/menc-feat-enc-libavcodec.html
mplayer-doc: /usr/share/doc/mplayer-doc/HTML/zh_CN/menc-feat-enc-libavcodec.html