s390/test_unwind: avoid build warning with W=1

Fix the following build warning with W=1

arch/s390/lib/test_unwind.c:172:21: warning: variable 'fops' set but not used [-Wunused-but-set-variable]
        struct ftrace_ops *fops;

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>