Depend explicitly on Gnulib ‘attribute’ module

Problem reported by Basil L. Contovounesios (bug#80428#8).
* admin/merge-gnulib (GNULIB_MODULES): Add ‘attribute’.

admin/merge-gnulib

@@ -29,7 +29,8 @@ export LC_ALL=C
 GNULIB_URL=https://git.savannah.gnu.org/git/gnulib.git
 
 GNULIB_MODULES='
-  alignasof alloca-opt binary-io bool boot-time builtin-expect byteswap
+  alignasof alloca-opt attribute
+  binary-io bool boot-time builtin-expect byteswap
   c-ctype c-strcase canonicalize-lgpl
   careadlinkat close-stream copy-file-range
   crypto/md5 crypto/md5-buffer

lib/gnulib.mk.in

@@ -83,6 +83,7 @@
 #  --avoid=wctype-h \
 #  alignasof \
 #  alloca-opt \
+#  attribute \
 #  binary-io \
 #  bool \
 #  boot-time \