hpet: Replace one-element array with flexible-array member

One-element arrays are deprecated[1] and are being replaced with
flexible array members in support of the ongoing efforts to tighten the
FORTIFY_SOURCE routines on memcpy(), correctly instrument array indexing
with UBSAN_BOUNDS, and to globally enable -fstrict-flex-arrays=3.

Replace one-element array with flexible-array member in struct hpet.

This results in no differences in binary output. The use of struct hpet
is never used with sizeof() and accesses via hpet_timers array are
already done after explicit bounds checking.

[1] https://github.com/KSPP/linux/issues/79

Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20221118034250.never.999-kees@kernel.org