Add btrfs.* to default PORTAGE_XATTR_EXCLUDE (527636)

In bug #527636 a user reported that install-xattr failed to copy
btrfs.compression attributes when doins was installing a file from
$FILESDIR. It is unlinkly that a package will have a valid use for
btrfs.* attributes when installing files to $D. Therefore, exclude
btrfs.* attributes in order to avoid reports like this in the
future.

X-Gentoo-Bug: 527636
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=527636
Acked-by: Brian Dolbec <dolsen@gentoo.org>