Add `.cache` to .gitignore.

The `.cache` directory seems to be created by clangd.

Bug: None
Change-Id: I2d43fd11dd13aaec6a0af00f9b7f1e72d73703f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/202900
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33045}
diff --git a/.gitignore b/.gitignore
index 8c7582c..8202b82 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,6 +26,7 @@
 *_proto_cpp.xml
 *~
 .*.sw?
+.cache
 .cipd
 .clangd
 .classpath