cmd/cgo: don't give inconsistent typedef error for cgo-defined types

The cgo tool predefines some C types such as C.uint. Don't give an
error if the type that cgo defines does not match the type in a header file.

Fixes #26743

Change-Id: I9ed3b4c482b558d8ffa8bf61eb3209415b7a9e3c
Reviewed-on: https://go-review.googlesource.com/127356


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>