The opus_headers.txt, etc. files are makefile
fragments defining variables with a list of
required header files for the build. As such
they should use the .mk filename extension,
just like opus_sources.mk, etc.

This file is included unconditionally by celt.h.
As such it's a required part of the build and
should be in the header index files.

their usage and to make any toolchain that doesn't like to #include
.c file happy.