runtime/cgo: add -Wno-nullability-completeness on Darwin

Fixes #35247

Change-Id: I4f2e243c89e9f745b82bcd181add87fad1443171
Reviewed-on: https://go-review.googlesource.com/c/go/+/205457
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/runtime/cgo/cgo.go

@@ -23,6 +23,9 @@ package cgo
 #cgo aix LDFLAGS: -Wl,-berok
 #cgo solaris LDFLAGS: -lxnet
 
+// Issue 35247.
+#cgo darwin CFLAGS: -Wno-nullability-completeness
+
 #cgo CFLAGS: -Wall -Werror
 
 #cgo solaris CPPFLAGS: -D_POSIX_PTHREAD_SEMANTICS