cmd/cgo: explicitly use void for functions with no parameters

[mauri870]

Currently, exported Go functions with no parameters generate C functions
with an empty parameter list. In C, a function with an empty parameter
list can accept any number of arguments, whereas a function with a single
void parameter explicitly declares that it takes no arguments.

To align the generated C functions with their Go prototypes, update the
code generation to explicitly include a void parameter for functions
with no parameters.

Fixes #68411

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Message from Mauri de Souza Meneguzzo:

Patch Set 2: Commit-Queue+1

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Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-12-24T12:32:10Z","revision":"ea8e7471cdcbeea9766334a6d65dafa306a895f1"}

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Message from Mauri de Souza Meneguzzo:

Patch Set 2: -Commit-Queue

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Message from Go LUCI:

Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-windows-amd64 has failed with summary (view all results):

---

• go on master (change 638635)

Build or test failure, click here for results.

To reproduce, try gomote repro 8727668705758681185.

Additional links for debugging:

• Output for go tool dist test -json [shard 1]

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Patch Set 2: LUCI-TryBot-Result-1

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Message from Mauri de Souza Meneguzzo:

Patch Set 3: Commit-Queue+1

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Patch Set 3:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-12-24T13:16:07Z","revision":"278cf67e4e90e232aee9b9baa84292b31e2d2272"}

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Message from Mauri de Souza Meneguzzo:

Patch Set 3: -Commit-Queue

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Patch Set 3:

This CL has passed the run

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Patch Set 3: LUCI-TryBot-Result+1

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