Don't export QtDeclarative symbols needed by QtCreator on Symbian

Changing private headers used by Qt Creator require declarative Symbian def files to be updated (tedious non-productive work), even though those private APIs are only exported for desktop platforms. So instead of having macro Q_DECLARATIVE_EXPORT in the headers required by QtCreator we should have specialized version called Q_DECLARATIVE_PRIVATE_EXPORT that only exports symbols on desktop platforms.

File qdeclarativeglobal_p.h:
#ifdef Q_OS_SYMBIAN
#define Q_DECLARATIVE_PRIVATE_EXPORT
#else
#define Q_DECLARATIVE_PRIVATE_EXPORT Q_DECLARATIVE_EXPORT
#endif

Headers:
-class Q_DECLARATIVE_EXPORT QDeclarativeMetaType
+class Q_DECLARATIVE_PRIVATE_EXPORT QDeclarativeMetaType