runtime/cgo: does not build with -Wdeclaration-after-statement [1.24 backport]

[gopherbot]

@ianlancetaylor requested issue #71961 to be considered for backport to the next 1.24 minor release.

@gopherbot Please open backport issues.

This breaks the SWIG CI build.

[gopherbot]

Change https://go.dev/cl/652936 mentions this issue: [release-branch.go1.24] runtime/cgo: avoid errors from -Wdeclaration-after-statement

[gopherbot]

Closed by merging CL 652936 (commit 45a5271) to release-branch.go1.24.

[gopherbot]

Change https://go.dev/cl/653137 mentions this issue: [release-branch.go1.24] runtime/cgo: avoid errors from -Wdeclaration-after-statement