handle_error, Gst::Caps, Gst::Iteratorfix simple errors: minor fixes

    * gstreamer/gstreamermm/handle_error.h: fixed header guard name.
    * gstreamer/src/caps.hg: removed warning with mixing struct/class in
      class definition/declaration.
    * gstreamer/src/iterator.hg: avoid using uninitialized memory.
Gst::Iterator: fix swap method

    * gstreamer/src/iterator.hg: initialized GValue object.