QtCore: avoid unnecessary export in static build

We only need it to be exported in shared builds.
Without this change, there will always be an
exported symbol called "qt_startup_hook" for
executables that linked against QtCore statically,
this is absolutely not the Qt user would expect.

Change-Id: Icf19df09531e13184ea019aa708d6b93fa626f85
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -241,7 +241,11 @@ void QCoreApplicationPrivate::processCommandLineArguments()
 
 // Support for introspection
 
-extern "C" void Q_DECL_EXPORT_OVERRIDABLE qt_startup_hook()
+extern "C" void
+#ifdef QT_SHARED
+Q_DECL_EXPORT_OVERRIDABLE
+#endif
+qt_startup_hook()
 {
 }