Link printer plugins into user projects when using a static Qt

When building a user project against a static Qt on Windows or macOS the
static printer support plugins were not linked.  Move the
Q_IMPORT_PLUGIN statements to qplatformprintplugin.cpp, which is a
translation unit that is guaranteed to be linked into the user project.

Pick-to: 6.2
Fixes: QTBUG-97490
Change-Id: Ic35c6483a664a06461304c7e8a2b7b06ce651ed6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/printsupport/kernel/qplatformprintplugin.cpp

@@ -48,6 +48,12 @@
 
 QT_BEGIN_NAMESPACE
 
+#if defined(Q_OS_MACOS)
+Q_IMPORT_PLUGIN(QCocoaPrinterSupportPlugin)
+#elif defined(Q_OS_WIN)
+Q_IMPORT_PLUGIN(QWindowsPrinterSupportPlugin)
+#endif
+
 Q_GLOBAL_STATIC_WITH_ARGS(QFactoryLoader, loader,
     (QPlatformPrinterSupportFactoryInterface_iid, QLatin1String("/printsupport"), Qt::CaseInsensitive))
 

src/printsupport/platform/macos/qcocoaprintersupport.mm

@@ -128,8 +128,6 @@ QPlatformPrinterSupport *QCocoaPrinterSupportPlugin::create(const QString &key)
     return new QCocoaPrinterSupport();
 }
 
-Q_IMPORT_PLUGIN(QCocoaPrinterSupportPlugin)
-
 #include "qcocoaprintersupport.moc"
 
 QT_END_NAMESPACE

src/printsupport/platform/windows/qwindowsprintersupport.cpp

@@ -103,8 +103,6 @@ QPlatformPrinterSupport *QWindowsPrinterSupportPlugin::create(const QString &key
     return nullptr;
 }
 
-Q_IMPORT_PLUGIN(QWindowsPrinterSupportPlugin)
-
 #include "qwindowsprintersupport.moc"
 
 QT_END_NAMESPACE