Add Xcode support.

Global/Xcode.gitignore

+build/
+*.pbxuser
+!default.pbxuser
+*.mode1v3
+!default.mode1v3
+*.mode2v3
+!default.mode2v3
+*.perspectivev3
+!default.perspectivev3
+xcuserdata
+*.xccheckout
+profile
+*.moved-aside
+DerivedData
+*.hmap
+*.ipa
+

Objective-C.gitignore

-# Xcode
-build/
-*.pbxuser
-!default.pbxuser
-*.mode1v3
-!default.mode1v3
-*.mode2v3
-!default.mode2v3
-*.perspectivev3
-!default.perspectivev3
-xcuserdata
-*.xccheckout
-profile
-*.moved-aside
-DerivedData
-*.hmap
-*.ipa
-
 # CocoaPods
 #
 # We recommend against adding the Pods directory to your .gitignore. However