Handle static inline GtkOrdering function

Handle static inline GtkOrdering function

The introspection scanner does not handle `static inline` functions:
they are not in the shared library, so cannot be dlsym() out of it; and
the `static` keyword tells g-ir-scanner to skip the function declaration
entirely.

We can trick the scanner into thinking the gtk_ordering_from_cmpfunc()
symbol is a real, public one, by declaring and defining a regular
function under the `__GI_SCANNER__` guard; the symbol does not appear
when actually building GTK, or any code using GTK, so we don't risk
collisions.