coreplugin: missing framework on OS X 10.9

Whatever the reason, libCore.dylib on OS X 10.9 and from Qt Creator git/head (commit 51bb521) no longer loads because it fails to find `NSMetadataQueryUpdateAddedItemsKey` in the Foundation framework.

A simple test shows that this symbol requires the CoreServices framework, and indeed libCore.dylib loads after adding that additional framework to coreplugin.pro and rebuilding.

Curiously my earlier build of Qt Creator 3.4.0 (release) continues to work without issues. I checked on a different Mac also running OS X 10.9.5 and having the latest updates installed: same story there, so I can exclude bitrot on my machine.