Add /.cache/ to .gitignore (for vscode clangd)

The vscode clangd extension now uses this directory.

Change-Id: I0232a3ac556ddbe2b867e8db508e1a86380fd6e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2297814Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: George Steel <gtsteel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789670}

.gitignore

@@ -71,6 +71,7 @@ vs-chromium-project.txt
 /android_emulator_sdk
 /ash/ash_unittests_run.xml
 /base/base_unittests_run.xml
+/.cache/
 # See build/.gitignore for entries covering src/build.
 
 # The Chrome OS build creates a /c symlink due to http://crbug.com/54866.