Include the AST.h to non-compiled files.

[ktopouzi]

When trying to add a new test in AST.h, I realized it is not the header that Test.AST.cs uses. but an external dependency that CppSharp.Parse.CLI has, while the correct one is under Tests/Native. So include the file to non-complied in AST test project may avoid confusion.

[tritao]

To do this you need to add some new Premake build code to https://github.com/mono/CppSharp/blob/master/src/Generator.Tests/premake5.lua#L9:

Something like this should work:

files { testsdir .. "Native/AST.h" }
filter "files:**.h"
   buildaction "None"
filter {}

Can you give it a try and send a PR?