Add build folder to .gitignore

[fel1x-developer]

By convention, many CMake users build their project in build folder in git repo. Although this is not the recommended way for HalfLife Unified SDK, this can prevent confusion and mistakes by people who are doing so.

[Mikk155]

i've configured cmake to build the project somewhere else and yet there are cache generated on project/build that i have to delete the folder every time i want to commit anything to a new branch (i don't want to update the .gitignore for every branch that directly inherits from here) so adding build/* to gitignore would be a great addition