misc/cgo/gmp: change pi.go to use standard Go copyright header

misc/cgo/gmp: change pi.go to use standard Go copyright header

This file is not part of the benchmark shootout, and we wrote it, so use
the usual copyright header, not a partial version of the shootout
license.

Fixes #13575.

Change-Id: Ib610e2ad82914b4ef096a2424cfffe3383db2d5b
Reviewed-on: https://go-review.googlesource.com/17715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>