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: NMartin Kletzander <mkletzan@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

m4/virt-attr.m4

@@ -5,7 +5,7 @@ AC_DEFUN([LIBVIRT_ARG_ATTR],[
 ])
 
 AC_DEFUN([LIBVIRT_CHECK_ATTR],[
-  LIBVIRT_CHECK_LIB([ATTR], [attr], [getxattr], [attr/xattr.h])
+  LIBVIRT_CHECK_LIB([ATTR], [attr], [getxattr], [sys/xattr.h])
 ])
 
 AC_DEFUN([LIBVIRT_RESULT_ATTR],[

tests/securityselinuxhelper.c

@@ -34,7 +34,7 @@
 #include <string.h>
 #include <sys/vfs.h>
 #include <unistd.h>
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 
 #ifndef NFS_SUPER_MAGIC
 # define NFS_SUPER_MAGIC 0x6969

tests/securityselinuxlabeltest.c

@@ -28,7 +28,7 @@
 
 #include <selinux/selinux.h>
 #include <selinux/context.h>
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 
 #include "internal.h"
 #include "testutils.h"