cmd/cgo: use full prototype for main in C code

cmd/cgo: use full prototype for main in C code

Avoids pedantic errors from modern C compilers.

Fixes #70769

Change-Id: Ibe0d9960e998eb0346b97d7bd69eb7de0d0e6c60
Reviewed-on: https://go-review.googlesource.com/c/go/+/635095
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>