Pull Request: fix vsg cmake macro "vsg_add_cmake_support_files"

Description

The macro vsg_add_cmake_support_files is missing an export( EXPORT ) CMake statement.

• results:
  • in-tree builds not working properly
  • FetchContent does not work with vsg
• fix: simply add corresponding CMake command.

Fixes # no issue created, simply fixed it.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Create a repo, use CMake FetchContent to get current vsg into the build. Even if all works out well, linking the executable / library to vsg::vsg FAILS.

Create a repo with submodule(s) vsg (and maybe vsgImGui), try to link own executable to vsg::vsg after including vsg via add_subdirectory() → FAILS.

In In-Tree case: this works directly

add_subdirectory( vsg )
export( EXPORT vsgTargets NAMESPACE vsg:: )

But it is not a "pro" solution, as this line of code belongs into that macro!

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
  • I have committed tests that show my fix is working