Add c_icu to gitignore

It seems c_icu symlink is also created along with /c symlink for
ChromeOS build (/c is for chromeos-chrome package, /c_icu is for
chrome-icu package).  It's better to ignore it for the same reason.

Bug: none
Test: build_packages with chromeos chroot environment
Change-Id: Iad0a4addac492580601a16faa26ec34ed2fe77e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425123Reviewed-by: Honglin Yu <honglinyu@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809698}

.gitignore

@@ -72,8 +72,9 @@ vs-chromium-project.txt
 /base/base_unittests_run.xml
 # See build/.gitignore for entries covering src/build.
 
-# The Chrome OS build creates a /c symlink due to http://crbug.com/54866.
+# The Chrome OS build creates a /c and /c_icu symlink due to http://crbug.com/54866.
 /c
+/c_icu
 /cdm
 /ceee/internal/
 /chrome/android/profiles/afdo.prof
@@ -327,4 +328,3 @@ vs-chromium-project.txt
 
 # Ignore the default results output directory for tools/run-swarmed.py
 /results
-