Fix include for xattr.h

Fix include for xattr.h

The proper file that should be included is `sys/xattr.h` as that comes from
`glibc` and not `attr/xattr.h` which ships with the `attr` utility.

We're most probably not the only ones because `attr/xattr.h` added a #warning to
their include resulting in the following compilation errors:

In file included from securityselinuxlabeltest.c:31:0:
/usr/include/attr/xattr.h:5:2: error: #warning "Please change your <attr/xattr.h> includes to <sys/xattr.h>" [-Werror=cpp]
 #warning "Please change your <attr/xattr.h> includes to <sys/xattr.h>"
  ^~~~~~~

In file included from securityselinuxhelper.c:37:0:
/usr/include/attr/xattr.h:5:2: error: #warning "Please change your <attr/xattr.h> includes to <sys/xattr.h>" [-Werror=cpp]
 #warning "Please change your <attr/xattr.h> includes to <sys/xattr.h>"
  ^~~~~~~

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>