Tweak test to work with both 6g and gccgo.

Tweak test to work with both 6g and gccgo.

Pull the struct into a single line, since 6g reports the error
on the last line of the struct definition and gccgo reports it
on the first line.

6g:
bug215.go:12: invalid recursive type A

gccgo:
bug215.go:12:6: error: invalid recursive type ‘A’

R=rsc
https://golang.org/cl/180044