windows: add GRN_API to internal functions for Visual Studio Express 2008

Redmine: refs the 21th comment on #1964
http://redmine.groonga.org/issues/1964#note-21

This change will solve the followings:

    1>ctx.obj : error LNK2019: ... _grn_com_recv ...
    1>ctx.obj : error LNK2019: ... _grn_com_init ...
    1>ctx.obj : error LNK2019: ... _grn_com_fin ...
    1>db.obj : error LNK2019: ... _grn_edge_dispatch ...
    1>db.obj : error LNK2019: ... _grn_edges_add_communicator ...

But it will not solve the followings:

    1>ctx.obj : error LNK2019: ... _grn_com_copen ...
    1>ctx.obj : error LNK2019: ... _grn_com_send ...
    1>ctx.obj : error LNK2019: ... _grn_com_close ...
    1>db.obj : error LNK2019: ... _grn_msg_open ...

Umm...