runtime/testdata/testprogcgo: fix TestCgoPanicCallback

A cgo file with "//export" declarations is not permitted to have function
definitions in the cgo comments.

Fixes #49188

Change-Id: I5c24b62b259871473ee984cea96a0edd7d42d23a
Reviewed-on: https://go-review.googlesource.com/c/go/+/359195


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>