Hide http-parser completely

[junfx]

As the cmake script written, libhttp-parser.a won't be installed, so maybe the header http-parser.h is not necessary to expose.

I guess this project is developed under windows with MSVC, which allows a static lib to be linked into another static lib. And This is what httpxx expected.
Most compilers do not allow this, MSVC is an exception.

If this is an issue, a possible solution is linking http-parser.o directly into libhttpxx.a.

[AndreLouisCaron]

Yes, this was originally developed on Windows. I'm OK with compiling http-parser.o directly into libhttpxx if you want to send in a PR.