fcntl.h: Add O_XATTR for Solaris compatibility

Commit 2ec2ba7e231d added O_NAMEDATTR for support
of the Solaris style extended attribute mechanism.

This patch adds O_XATTR as a synonym so that
software that has Solaris extended attribute support
can be built easily.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51099