[UnitTests] Add the libllbuild header search path to the new target for C API unit tests
[UnitTests] Add the libllbuild header search path to the new target for C API unit tests

This is necessary in order for them to find the C API headers.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild