qtmultimedia: Fix crash when use Gstreamer 0.10 plugin

In QVideoSurfaceGstSink::get_caps(), when use 0.10 version Gstreamer,
It create GstCap with a wrong type (G_TYPE_STRING) of parameter 'format',
a g_strcpy exception will be found.
This patch fixes the parameter type to use GST_TYPE_FOURCC as expacted.

Signed-off-by: Mingtao Qu <zarelaky@hotmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>