Avoid implicit gcc nonnull attribute in vwarnx().

We removed the nonnull attributes from our headers long ago, but still
__printflike() includes it implicitly. This will cause the NULL check to
be optimized away in higher -O levels and it will also trigger a
-Wnonnull-compare warning.

Avoid warning with it in vwarnx().

Obtained from: DragonfLyBSD (git 6329e2f68af73662a1960240675e796ab586bcb1)