Add MODULE.bazel.lock to gitignore. (#509)

I would add --lockfile_mode=off to .bazelrc, but this should support older versions of bazel before the lockfile_mode flag was released.
diff --git a/.gitignore b/.gitignore
index a6ef824..a2e7adc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /bazel-*
+MODULE.bazel.lock
\ No newline at end of file