Ignore Cargo.lock in .gitignore

Since the merge of #2146, `Cargo.lock` is no longer checked in the git
repository, but `cargo` generates it anyway, if we are not in the main rust
repository. Ignore it to avoid eventual confusion when contributing directly
to clippy.

Author: MaloJaffre

.gitignore

@@ -13,6 +13,7 @@ out
 *.exe
 
 # Generated by Cargo
+Cargo.lock
 /target/
 /clippy_lints/target/