[PATCH] wmphoto: Make the wmp_decoder_*() functions static.

[Esme Povirk (she/they)]

The decoder interface is supposed to be the same on the win32 side and
the unixlib side. Currently, we don't have any encoders or decoders
implemented on the win32 side. Strictly speaking, I don't think we
need CDECL on the unixlib side, but I'd like to keep them so the
implementations carry over easily to win32.

It'd be nice to port GIF, BMP, and TGA support to this system so we
can share more code between them, but it's probably not worth the time
since they're already implemented. We may also want to eliminate the
unixlib entirely and move all our decoders to the win32 side.