Fixed symbol visibility issues with explicit instantiation

[Idclip]

Signed-off-by: Nick Avramoussis [email protected]

Closes #1272

[e4lam]

~Have we tested using this on Windows? I'm missing how an OpenVDB DLL user can link against the instantiations if they're no longer declared as dllimport.~

EDIT: Ah I missed that we're now directly doing the import/exports of the instantiations. But perhaps it worries me a bit that we're redeclaring them with the same macro names.

[Idclip]

@danrbailey bumping this

EDIT: Ah I missed that we're now directly doing the import/exports of the instantiations. But perhaps it worries me a bit that we're redeclaring them with the same macro names.

Happy to use different names for the import/export specs