Fix cmake error when build with capstone as submodule

[WerWolv]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

Hey, really simple change making sure that cpack doesn't try to look for a LICENSE.TXT file in the main project root but in the current project root. This fixes the usecase where capstone is being included as a submodule and loaded using add_subdirectory in a cmake project. Previously it would look for LICENSE.TXT in the main project root directory, now it's looking for it in the capstone subdirectory.

Test plan

• Create a sample cmake project, put capstone into a subfolder and include it in the main cmake script using add_subdirectory. Previously this would fail with a cpack error. Now it doesn't anymore.

Closing issues

• None