gnulib: fix getopt build with glibc
    
    From gnulib commit
    
      commit e3461d1c21a99bcef1b8826f710434e0ffb5adea
      Author: Paul Eggert <eggert@cs.ucla.edu>
      Date:   Sun Jun 11 15:53:09 2017 -0700
    
        getopt-posix: port to glibc 2.25.90
    
        Problem reported by Daniel P. Berrange in:
        http://lists.gnu.org/archive/html/bug-gnulib/2017-06/msg00003.html
        * lib/getopt-pfx-core.h (_GETOPT_CORE_H):
        * lib/getopt-pfx-ext.h (_GETOPT_EXT_H):
        #undef if __GETOPT_PREFIX is defined.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>