posix: Fix test-errno build with fortify enable

With gcc 11.3.1, building with -D_FORTIFY_SOURCE=2 shows:

In function ‘getgroups’,
    inlined from ‘do_test’ at test-errno.c:129:12:
../misc/sys/cdefs.h:195:6: error: argument 1 value -1 is negative
[-Werror=stringop-overflow=]
  195 |    ? __ ## f ## _alias (__VA_ARGS__)
      \
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../posix/bits/unistd.h:115:10: note: in expansion of macro
‘__glibc_fortify’
  115 |   return __glibc_fortify (getgroups, __size, sizeof (__gid_t),
      |          ^~~~~~~~~~~~~~~
../posix/bits/unistd.h: In function ‘do_test’:
../posix/bits/unistd-decl.h:135:28: note: in a call to function
‘__getgroups_alias’ declared with attribute ‘access (write_only, 2, 1)’
  135 | extern int __REDIRECT_NTH (__getgroups_alias, (int __size,
      __gid_t __list[]),
      |                            ^~~~~~~~~~~~~~~~~
../misc/sys/cdefs.h:264:6: note: in definition of macro ‘__REDIRECT_NTH’
  264 |      name proto __asm__ (__ASMNAME (#alias)) __THROW

It builds fine with gcc 12 and gcc 13.

Checked on x86_64-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>