entity.h is not exposed as a public header

[dominickpastore]

I'm not sure if this is intentional, but entity.h is not exposed as a public header in src/CMakeLists.txt.

I suppose this makes sense, since entity.c is only built as part of MD4C-HTML, and users of that library don't need entity.h directly. But maybe it would be a good idea to make entity.c/.h available to applications that need to implement their own renderer.