[Chromecast] Add libassistant to .gitignore

BUG=None
TEST=None

Change-Id: Id8697dae440df7f2d247dc06d69ba5c424ea50a6
Review-Url: https://codereview.chromium.org/2799943003
Cr-Commit-Position: refs/heads/master@{#462715}

.gitignore

@@ -184,6 +184,7 @@ vs-chromium-project.txt
 /ios/third_party/material_text_accessibility_ios/src
 /ios/third_party/ochamcrest/src
 /ios_internal
+/libassistant
 /llvm
 /media/cast/logging/cast_logging_proto_lib.xml
 /media/cdm/api