fs_close() closes the connection to a file system which opened with
either fs_set_blk_dev() or fs_set_dev_with_part(). Many file system
functions implicitly call fs_close(), e.g. fs_closedir(), fs_exist(),
fs_ln(), fs_ls(), fs_mkdir(), fs_read(), fs_size(), fs_write()
and fs_unlink().
So just export it.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>