Doc: Add required include statement for qmlRegisterSingletonType docs
Doc: Add required include statement for qmlRegisterSingletonType docs

Adds a missing include for QtQml into QQmlEngine class reference code
snippets.

Task-number: QTBUG-30642
Change-Id: I2ec10de5b31b4854d5aaea08987b9d75926d2ef4
Reviewed-by: Jerome Pasion <c10995d24f6b454503389e8d00d837aef038985d@digia.com>

qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,corngood/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,corngood/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,corngood/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,corngood/qtdeclarative,matthewvogt/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative