cmd/cgo: revert CL 49490 "fix for function taking pointer typedef"

CL 49490 fixed a warning when compiling the C code generated by cgo,
but it introduced typedef conflicts in Go code that cgo is supposed to
avoid.

Original CL description:

    cmd/cgo: fix for function taking pointer typedef

    Fixes #19832

Updates #19832
Fixes #23720

Change-Id: I22a732db31be0b4f7248c105277ab8ee44ef6cfb
Reviewed-on: https://go-review.googlesource.com/92455


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>