add needed include for qdatastream

[whyrusleeping]

Just got my lightpack and the application failed to build due to a missing include (i think its an issue caused by a newer version of qt). This fixed the build for me, figured i'd share.

Error i was experiencing:

qtsingleapplication/src/qtlocalpeer.cpp: In member function ‘bool QtLocalPeer::sendMessage(const QString&, int)’:
qtsingleapplication/src/qtlocalpeer.cpp:160:19: error: variable ‘QDataStream ds’ has initializer but incomplete type
     QDataStream ds(&socket);