gitignore: ignore xcode workspaces and projects

Calling ./configure --xcode creates xcode projects and a
workspace for io.js.

PR-URL: #1562
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>

.gitignore

@@ -76,3 +76,7 @@ deps/zlib/zlib.target.mk
 tools/faketime
 icu_config.gypi
 test.tap
+
+# Xcode workspaces and project folders
+*.xcodeproj
+*.xcworkspace