perf/x86: Annotate struct bts_buffer::buf with __counted_by()

Add the __counted_by() compiler attribute to the flexible array member
buf to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250305123134.215577-2-thorsten.blum@linux.dev