[PATCH] Fix OAuth feature detection on OpenBSD+Meson

Hi all,

I noticed that the OpenBSD build in CI wasn't running the libcurl
tests. Turns out the feature test I added in b0635bfda is subtly
broken, because it uses cc.check_header() rather than cc.has_header().
On OpenBSD, apparently, the <sys/event.h> header can't be compiled
without including additional prerequisite headers.

The attached patch fixes that by just switching to has_header(). (I
could add the prerequisites to the test instead, but I'd prefer to
exactly match the logic we use to determine the value of the
HAVE_SYS_EVENT_H macro. I don't think I had a good reason not to in
the first place.)

As a potential follow-up, is there any interest in switching the
Cirrus Meson setup to explicitly enable the features that we expect to
test? That would match what we do for Autoconf, I think; it would have
helped me catch my mistake earlier.

Thanks,
--Jacob