.gitignore: add third_party/libFuzzer

.gitignore was forgotten when adding a new dependency to
setup_links.py (https://codereview.webrtc.org/1857673002).

TBR=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1853113002 .

Cr-Commit-Position: refs/heads/master@{#12216}
diff --git a/.gitignore b/.gitignore
index 99dab8d..30b566c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -100,6 +100,7 @@
 /third_party/libc++-static
 /third_party/libc++abi
 /third_party/libevent
+/third_party/libFuzzer
 /third_party/libjingle
 /third_party/libjpeg
 /third_party/libjpeg_turbo