Followup to CL 17716, which updated cgo's boilerplate prologue code to
use standard C's _Complex instead of GCC's __complex extension.

Change-Id: I74f29b0cc3d13cab2853441cafbfe77853bba4f9
Reviewed-on: https://go-review.googlesource.com/17820
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>