misc/cgo/test: remove unnecessary forward declaration

This test otherwise fails to build on windows/arm64 as of CL 364774
due to a warning (promoted to an error) about a mismatched dllexport
attribute. Fortunately, it seems not to need the forward-declared
function in this file anyway.

Updates #49633
Updates #49721

Change-Id: Ia4698b85077d0718a55d2cc667a7950f1d8e50ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/366075


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>