kvm tools: Add __DECLARE_FLEX_ARRAY() in include/linux/stddef.h

Latest x86 UAPI headers uses __DECLARE_FLEX_ARRAY() macro so let us take
this macro from Linux UAPI header and add it to include/linux/stddef.h.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20230712163501.1769737-2-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>