Fix QT_NO_SYSTEMTRAYICON build.

The ifndef was added to the header but was missing in
the implementation file.

Change-Id: I25111b34b9c087465306ef81c23d6667257b5e71
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/gui/kernel/qplatformsystemtrayicon_qpa.cpp

@@ -42,6 +42,8 @@
 
 #include "qplatformsystemtrayicon.h"
 
+#ifndef QT_NO_SYSTEMTRAYICON
+
 QT_BEGIN_NAMESPACE
 
 /*!
@@ -159,3 +161,5 @@ QPlatformSystemTrayIcon::~QPlatformSystemTrayIcon()
 QT_END_NAMESPACE
 
 #include "moc_qplatformsystemtrayicon.cpp"
+
+#endif // QT_NO_SYSTEMTRAYICON