S390: Fix werror=unused-variable in ifunc-impl-list.c.

If the architecture level set is high enough, no IFUNCs are used at
all and the variable i would be unused.  Then the build fails with:
../sysdeps/s390/multiarch/ifunc-impl-list.c: In function ‘__libc_ifunc_impl_list’:
../sysdeps/s390/multiarch/ifunc-impl-list.c:76:10: error: unused variable ‘i’ [-Werror=unused-variable]
   76 |   size_t i = max;
      |          ^
cc1: all warnings being treated as errors