mumble_plugin.h: add MUMBLE_PLUGIN_EXPORT macro.
mumble_plugin.h: add MUMBLE_PLUGIN_EXPORT macro.

This allows plugins to use MUMBLE_PLUGIN_EXPORT
instead of __declspec(dllexport) -- but get the
same behavior on MSVC/Windows.

This is part of the effort to allow Windows PA
plugins to work with Wine games on Linux.

Lartza/mumble,Lartza/mumble,Lartza/mumble,LuAPi/mumble,Lartza/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble,LuAPi/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble