tools/security: Adjust secpol_tool.c for change to xc_interface_open

tools/security: Adjust secpol_tool.c for change to xc_interface_open

xc_interface_open() was called with improper number of arguments. It
is fixed by this patch.

This appears to have been missed by 21483:779c0ef9682c. The interface
change also included the return type (int->xc_interface *) but that
was already covered in 21483.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>