x86/platform/uv: Annotate struct uv_rtc_timer_head with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Found with Coccinelle:

  https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci



Add __counted_by for struct uv_rtc_timer_head.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175151.work.118-kees@kernel.org