core: don't export unnecessary symbol in static build

User binary has this export when Qt is build statically,
we should only export it in shared Qt builds.

Change-Id: Iaa08e2a668bee6f094a49f20a0033c5eecdc1cbc
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qfloat16.cpp

@@ -13,7 +13,7 @@
 QT_BEGIN_NAMESPACE
 
 #if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
-Q_DECL_EXPORT int qRegisterNormalizedMetaType_qfloat16(const QByteArray &)
+Q_CORE_EXPORT int qRegisterNormalizedMetaType_qfloat16(const QByteArray &)
 {
     return QMetaType::Float16;
 }