gitignore: Ignore some Rust/Cargo generated files

Although the Cmake rules for Rust leave all build files in the build
directory, sometimes tools such as IDEs can leave a few files in the
source tree.  Ignore these to make sure they don't get committed.

Signed-off-by: David Brown <david.brown@linaro.org>

.gitignore

@@ -59,6 +59,13 @@ venv
 .clangd
 new.info
 
+# Cargo drops lock files in projects to capture resolved dependencies.
+# We don't want to record these.
+Cargo.lock
+
+# Cargo encourages a .cargo/config.toml file to symlink to a generated file.  Don't save these.
+.cargo/
+
 # CI output
 compliance.xml
 _error.types