gnulib: Bring back the fix for resolving unused variable issue

This patch resolved a minor issue spotted by Coverity:
  a983d36bd9 (gnulib/regexec: Resolve unused variable)

But, it was removed by the Gnulib update:
  2b79024598 (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 a983d36bd9 (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>

bootstrap.conf

@@ -87,7 +87,8 @@ bootstrap_post_import_hook () {
   for patchname in fix-width \
       fix-regcomp-resource-leak \
       fix-regexec-resource-leak \
-      fix-gcc-15-compile; do
+      fix-gcc-15-compile \
+      fix-unused-value; do
     patch -d grub-core/lib/gnulib -p2 \
        < "grub-core/lib/gnulib-patches/$patchname.patch"
   done