Do not use an export macro in the function definition

qmlExecuteDeferred is exported using Q_QML_EXPORT in
qqml.h, so we don't need to use one here. Better not have any
than have the wrong one, which can cause problems.

Cherry picked from d6d2074ff in qtquick1
Change-Id: Ief978feb503b7381ed462f71ebbd7986bd6862c3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>