runtime/cgo: avoid errors from -Wdeclaration-after-statement
It's used by the SWIG CI build, at least, and it's an easy fix.
Fixes #71961
Change-Id: Id21071a5aef216b35ecf0e9cd3e05d08972d92fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/652181 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
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