perf/x86/intel/pt: Annotate 'pt_cap_group' with __ro_after_init

perf/x86/intel/pt: Annotate 'pt_cap_group' with __ro_after_init

'pt_cap_group' is written to in pt_pmu_hw_init() and not modified after.
This makes it a suitable candidate for annotating as __ro_after_init.

Signed-off-by: Zubin Mithra <zsm@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Link: http://lkml.kernel.org/r/20180912164510.23444-1-zsm@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>