Add util/bot/libFuzzer to .gitignore.

The gclient-managed files usually go in .gitignore. I think without it,
we have to redownload it all the time on the bots? Though this also
makes my git status cleaner.

Change-Id: Ic9bac6796bd3dcdea5039bcac39e853d9f12906b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46824
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

.gitignore

@@ -15,6 +15,7 @@ util/bot/cmake-mac
 util/bot/cmake-win32
 util/bot/cmake-win32.zip
 util/bot/golang
+util/bot/libFuzzer
 util/bot/libcxx
 util/bot/libcxxabi
 util/bot/llvm-build