Make macros more portable with `$crate`
Follows !581 (comment 649476)
Libraries using the gst_plugin_define
and gst_error_msg
macros may or may not import other macros or namespaces into their scope; reuse what is already imported in gstreamer
s root (following existing code) to be less dependent on the surrounding scope.
Note that these are just what I ran into while developing my own plugin, I did not check other macros for similar issues.