selftests: timers: freq-step: fix compile error

Fix compile error due to ksft_exit_skip() update to take var_args.

freq-step.c: In function ‘init_test’:
freq-step.c:234:3: error: too few arguments to function ‘ksft_exit_skip’
   ksft_exit_skip();
   ^~~~~~~~~~~~~~
In file included from freq-step.c:26:0:
../kselftest.h:167:19: note: declared here
 static inline int ksft_exit_skip(const char *msg, ...)
                   ^~~~~~~~~~~~~~
<builtin>: recipe for target 'freq-step' failed

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>