src/interface.h: Cleanup structure definitions

Rather than doing something like

    struct my_struct {
        int member;
    } my_struct;

just do

    struct my_struct {
        int member;
    };

I'm not really sure what the first one means, it doesn't seem to do any
harm, but it's probably wrong to do it or it's just superfluous at the
lest.

Signed-off-by: Andrew Clayton <andrew@opentechlabs.co.uk>