gnulib regex.h should be updated

[psumbera]

While building version 4.9.3 on Solaris I see:

/builds/psumbera/userland-lftp-4.9.3/components/lftp/lftp-4.9.3/lib/regex.h:682:64: error: use of parameter outside function body before ']' token
  682 |                                         _REGEX_NELTS (__nmatch)],
      |                                                                ^
/builds/psumbera/userland-lftp-4.9.3/components/lftp/lftp-4.9.3/lib/regex.h:682:65: error: expected ')' before ',' token
  682 |                                         _REGEX_NELTS (__nmatch)],
      |                                                                 ^
      |                                                                 )
/builds/psumbera/userland-lftp-4.9.3/components/lftp/lftp-4.9.3/lib/regex.h:679:20: note: to match this '('
  679 | extern int regexec (const regex_t *_Restrict_ __preg,
      |                    ^
/builds/psumbera/userland-lftp-4.9.3/components/lftp/lftp-4.9.3/lib/regex.h:683:21: error: expected unqualified-id before 'int'
  683 |                     int __eflags);
      |                     ^~~

This can be resolved via:

https://github.com/coreutils/gnulib/commit/500d839fdfbc507e53ca1d3cf502e6b190b0fcf5