Add util/bot files to .gitignore.

bot_update does a git clean -dff before each run, so we were
redownloading all the utilities on each run. This should make the bots
only download them when the change. (Chromium's setup is similar.)

Change-Id: I7eb83217761ceabe58b5480242a7df93d9bfaa52
Reviewed-on: https://boringssl-review.googlesource.com/12561
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

.gitignore

@@ -4,3 +4,17 @@ ssl/test/runner/runner
 *.swo
 doc/*.html
 doc/doc.css
+
+util/bot/android_tools
+util/bot/cmake-linux64
+util/bot/cmake-linux64.tar.gz
+util/bot/cmake-mac
+util/bot/cmake-mac.tar.gz
+util/bot/cmake-win32
+util/bot/cmake-win32.zip
+util/bot/gyp
+util/bot/llvm-build
+util/bot/perl-win32
+util/bot/perl-win32.zip
+util/bot/win_toolchain.json
+util/bot/yasm-win32.exe