build: update gnulib submodule to latest

Gnulib issymlinkat change reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2025-12/msg00044.html
* bootstrap: Update from Gnulib.
* src/copy.c: Include issymlinkat.h, not issymlink.h.

bootstrap

@@ -37,7 +37,7 @@ medir=`dirname "$me"`
 
 # A library of shell functions for autopull.sh, autogen.sh, and bootstrap.
 
-scriptlibversion=2025-11-12.14; # UTC
+scriptlibversion=2025-12-04.19; # UTC
 
 # Copyright (C) 2003-2025 Free Software Foundation, Inc.
 #
@@ -861,7 +861,8 @@ autopull()
     elif check_exists git-merge-changelog; then
       echo "$0: initializing git-merge-changelog driver"
       git config merge.merge-changelog.name 'GNU-style ChangeLog merge driver'
-      git config merge.merge-changelog.driver 'git-merge-changelog %O %A %B "%Y"'
+      git config merge.merge-changelog.driver \
+          'git-merge-changelog %O %A %B "%Y"'
     else
       echo "$0: consider installing git-merge-changelog from gnulib"
     fi

src/copy.c

@@ -44,7 +44,7 @@
 #include "hash.h"
 #include "hashcode-file.h"
 #include "ignore-value.h"
-#include "issymlink.h"
+#include "issymlinkat.h"
 #include "quote.h"
 #include "renameatu.h"
 #include "root-uid.h"