avcodec/libvpxenc.c: Fix linking with codec interface with msvc.

Default libvpx msvc buils used module definition to export symbols.
These symbols are exported as pointers to data which dont link when
referenced directly.

Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>