Use the include-what-you-use program to (partially) clean up header
includes, so that only includes which are needed, and no includes that
are not needed (or at least closer to that ideal) are done.
For a c-language project, the build-time improvements from this change
is minimal. This would have a much bigger impact on a C++ project than a
C-project for build times.
So for coturn, this change is mostly intended to just provide
consistency and make it easier to locate weird issues like strange
dependencies, and unnecessary connections between code.
