perf/UAPI: Clearly mark __PERF_SAMPLE_CALLCHAIN_EARLY as internal use

Vince noted that commit:

  6cbc304f ("perf/x86/intel: Fix unwind errors from PEBS entries (mk-II)")

'leaked' __PERF_SAMPLE_CALLCHAIN_EARLY into the UAPI namespace. And
while sys_perf_event_open() will error out if you try to use it, it is
exposed.

Clearly mark it for internal use only to avoid any confusion.

Requested-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>