Fix the build of scandir_b with GCC.

Use explicit typedefs for block thunk structures as in r264143.

Reviewed by: kib, adrian
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26256

(cherry picked from commit d10af81d2defe43db0db0a7dca3eefb047bd8ddc)