Hide *.xcworkspace files

For hiding xcworkspace files, along with xcodeproj files, both of which
are created by the ninja xcode generator

BUG=NONE
NOTRY=True

Review-Url: https://codereview.webrtc.org/2045733005
Cr-Commit-Position: refs/heads/master@{#13075}
diff --git a/.gitignore b/.gitignore
index 30e8d7f..8c5a145 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,7 @@
 *.vpwhistu
 *.vtg
 *.xcodeproj
+*.xcworkspace
 *_proto.xml
 *_proto_cpp.xml
 *~