kselftests: timers: freq-step: Define ADJ_SETOFFSET if device has older kernel headers

On some systems, the kernel headers haven't been updated to include
ADJ_SETOFFSET, so define it in the test if needed.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>

tools/testing/selftests/timers/freq-step.c

@@ -33,6 +33,10 @@
 #define MAX_FREQ_ERROR 10e-6
 #define MAX_STDDEV 1000e-9
 
+#ifndef ADJ_SETOFFSET
+  #define ADJ_SETOFFSET 0x0100
+#endif
+
 struct sample {
 	double offset;
 	double time;