build, qt: Make QWindowsVistaStylePlugin available again (regression)

In Qt 5.12.x style plugins are separated.

Co-authored-by: Jarol Rodriguez <jarolrod@tutanota.com>

src/qt/bitcoin.cpp

@@ -60,6 +60,7 @@
 Q_IMPORT_PLUGIN(QXcbIntegrationPlugin);
 #elif defined(QT_QPA_PLATFORM_WINDOWS)
 Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin);
+Q_IMPORT_PLUGIN(QWindowsVistaStylePlugin);
 #elif defined(QT_QPA_PLATFORM_COCOA)
 Q_IMPORT_PLUGIN(QCocoaIntegrationPlugin);
 Q_IMPORT_PLUGIN(QMacStylePlugin);