ICU-22604 Use 'void' for empty C parameter lists (-Wstrict-prototypes)

[roubert]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22604
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[FrankYFTang]

could you document how you generate this PR by tool so we can learn from you.

[roubert]

could you document how you generate this PR by tool so we can learn from you.

There's no tool for this, I simply used the warnings output by the compiler as input to old-fashioned search-and-replace.