gnulib: Bring back the fix for resolving unused variable issue

gnulib: Bring back the fix for resolving unused variable issue

This patch resolved a minor issue spotted by Coverity:
  a983d36bd917 (gnulib/regexec: Resolve unused variable)

But, it was removed by the Gnulib update:
  2b7902459803 (Update gnulib version and drop most gnulib patches)

It caused Coverity to continue to flag the issue. Daniel Kiper
suggested to bring back the patch a983d36bd917 (gnulib/regexec: Resolve
unused variable).

Fixes: CID 292459
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>