clang: don't define nocall

clang: don't define nocall

Clang doesn't support attribute error, and the possible equivalents
like diagnose_if don't seem to work well in this case as they trigger
when when the function is not called (just by being used by the
APPEND_CALL macro).

Define nocall to a noop on clang until a proper solution can be found.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
[jb: error -> __error__]
Acked-by: Jan Beulich <jbeulich@suse.com>