This patch fixes a warning "tst-unique4lib.cc:17:12: warning: 'b'
defined but not used [-Wunused-variable]". I'm not sure exactly what
aspects of the test are or are not significant for the issue it is
testing for; the patch makes the minimal change of marking the
variable with __attribute__ ((used)).
Tested for x86_64.
* elf/tst-unique4lib.cc (b): Mark with __attribute__ ((used)).