MDEV-36729: ha_example::show_func_example is incorrectly defined

[grooverdan]

• The Jira issue number for this PR is: MDEV-36729

Description

In the main.plugin this function is called assuming the function prototype int (*)(THD *, st_mysql_show_var *, void *, system_status_var *, enum_var_type)' as changed in b4ff645.

We update the ha_example::show_func_example to match the prototype on which it is called.

Release Notes

internal test issue.

How can this PR be tested?

Under clang UBSAN tests run the unit test main.plugin.

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.
• *This is a bug fix of a type than cannot be filtered with UBSAN filters that would otherwise block testing on the 10.6 branch with an updated UBSAN tester (MDBF-741).

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[montywi]

Ok to push