cargo: track lock file

To enable reproducible builds (with Nix), Cargo.lock needs to be tracked
in version control.

Author: lbeierlieb

vmicore/rust_src/.gitignore

@@ -1,2 +1,4 @@
 /target/
-Cargo.lock
+# For reproducible builds with Nix, we must check the file in, although it is a
+# library.
+# Cargo.lock