[fuchsia] Add qemu to third_party/.gitignore.

A new directory "third_party/qemu" was added to the checkout as of
https://chromium-review.googlesource.com/1178975 . This CL adds "qemu"
to the .gitignore file to keep Git from thinking it's an
untracked directory.

TBR=sergeyu@chromium.org

Change-Id: I6db095ce30dd13268f3720de7aa86a48920ed481
Reviewed-on: https://chromium-review.googlesource.com/1182183Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584567}

third_party/.gitignore

@@ -178,6 +178,7 @@
 /python_26
 /pywebsocket/src
 /pywebsocket/src
+/qemu
 /quic_trace/src
 /r8/lib
 /re2/src