fcntl.h: Add O_XATTR for Solaris compatibility

[rmacklem]

Summary

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.

Test Plan

Built bash in ports and saw that the "cd -@ <file>"
syntax worked without having to make any changes
to the port.