[Perl/perl5] 0cd472: embed.fnc: mark uvuni_to_utf8() as a mathom

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 0cd4721fc41a5e79770dc62d1a6800c17e785cad
      
https://github.com/Perl/perl5/commit/0cd4721fc41a5e79770dc62d1a6800c17e785cad
  Author: Tony Cook <t...@develop-help.com>
  Date:   2023-07-04 (Tue, 04 Jul 2023)

  Changed paths:
    M embed.fnc
    M embed.h
    M proto.h

  Log Message:
  -----------
  embed.fnc: mark uvuni_to_utf8() as a mathom

This was causing builds with -DNO_MATHOMS to fail on Win32,
makedef.pl would generate an export, but since the definition
is in mathoms.c linking would fail.