Fix ifunc-impl-list.c build for s390

Builds for s390 recently started failing with:

../sysdeps/s390/multiarch/ifunc-impl-list.c: In function '__libc_ifunc_impl_list':
../sysdeps/s390/multiarch/ifunc-impl-list.c:83:21: error: unused variable 'dl_hwcap' [-Werror=unused-variable]
   83 |   unsigned long int dl_hwcap = features->hwcap;
      |                     ^~~~~~~~

https://sourceware.org/pipermail/libc-testresults/2023q1/010855.html

Add __attribute__ ((unused)) as already done for another variable
there.

Tested with build-many-glibcs.py (compilers and glibcs) for
s390x-linux-gnu and s390-linux-gnu.

Note: s390x-linux-gnu-O3 started failing with a different error
earlier; that problem may still need to be fixed after this fix is in.

https://sourceware.org/pipermail/libc-testresults/2023q1/010829.html