Fixed QtMultimedia core namespace compilation.

Change-Id: Ie276a7766bd26f3e49b2bc9c1a28ec96f1e3e4b0
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/gsttools_headers/qgstvideobuffer_p.h

@@ -58,6 +58,8 @@
 
 #include <gst/gst.h>
 
+QT_BEGIN_NAMESPACE
+
 class QGstVideoBuffer : public QAbstractVideoBuffer
 {
 public:
@@ -79,5 +81,6 @@ private:
     QVariant m_handle;
 };
 
+QT_END_NAMESPACE
 
 #endif