linux/types.h: Fix definition of uintcap_t

__kernel_uintcap_t is an arm64-specific uapi type, so it does not
make sense to use it to define uintcap_t. Besides, uintcap_t is only
defined if CHERI is available, so defining it as __uintcap_t
directly is perfectly fine.

Fixes: ("linux/types.h: Introduce uintcap_t")
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>