build: Include sys/cdefs.h in our fcntl.h shim

On Linux with musl, sys/cdefs.h isn't included with fcntl.h, so when we
use BEGIN_DECL and END_DECL in this file, it fails. There's no harm
in unconditionally including sys/cdefs.h here, so do that to avoid
encoding exactly where it is or isn't needed so we don't have to know
too much about the internal state of other libc implementations.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45351