src: don't mark addon_register_func as dllimport

addon_register_func and its cousin addon_context_register_func are type
definitions, dllimport and dllexport are name mangling directives, i.e.
they're quite unrelated concepts.  MinGW complains about mixing them
when cross-compiling native add-ons.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Bert Belder <bertbelder@gmail.com>