cmake: clang: Disable deprecated non-prototype warning

Clang 15 added a new warning type `-Wdeprecated-non-prototype` that
warns about the functions without prototypes, which have been
deprecated since the C89 and will not work in the upcoming C2x.

This commit disables the warning because Zephyr deliberately makes use
of the functions without prototypes to allow the use of a "generic"
function pointer (notoriously in the cbprintf implementation) and
Zephyr will not move to the C2x in the foreseeable future.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>