We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
This can be used in a generic way as common interface by all platforms
that, in one way or another, pass around physical memory addresses.
This is used by the gl lib and seems useful enough, so might just as
well move it next to the other allocators.
https://bugzilla.gnome.org/show_bug.cgi?id=779067
