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.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/events/intel/pt.c

@@ -95,7 +95,7 @@ static ssize_t pt_cap_show(struct device *cdev,
 	return snprintf(buf, PAGE_SIZE, "%x\n", pt_cap_get(cap));
 }
 
-static struct attribute_group pt_cap_group = {
+static struct attribute_group pt_cap_group __ro_after_init = {
 	.name	= "caps",
 };