gitignore: Remove Cargo.lock

Cargo.lock will need to be checked in to ensure reproducible builds.
However continue to ignore Cargo.lock in subdirectories as the main Cargo.lock
shouls dictate the versions of everything.

Change-Id: I448527b3c23bb9de939682ec1a78fd06a2530857
Signed-off-by: Dylan Reid <dgreid@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/538067
Reviewed-by: Zach Reizner <zachr@chromium.org>
diff --git a/.gitignore b/.gitignore
index b6d5cb1..6ead6aa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 target/
 **/*.rs.bk
-Cargo.lock
 **/*.sw[po]
 **/*.orig
+**/Cargo.lock
+!/Cargo.lock