Change Cretonne's license to "Apache-2.0 WITH LLVM-exception".

This adds the "LLVM-exception" to Cretonne's existing Apache-2.0
license.

https://spdx.org/licenses/LLVM-exception.html

lib/filetests/Cargo.toml

@@ -3,7 +3,7 @@ name = "cretonne-filetests"
 authors = ["The Cretonne Project Developers"]
 version = "0.13.0"
 description = "Test driver and implementations of the filetest commands"
-license = "Apache-2.0"
+license = "Apache-2.0 WITH LLVM-exception"
 documentation = "https://cretonne.readthedocs.io/en/latest/testing.html#file-tests"
 repository = "https://github.com/cretonne/cretonne"
 publish = false