build: fix build with libselinux 2.3
    
    Several function signatures changed in libselinux 2.3, now taking
    a 'const char *' instead of 'security_context_t'.  The latter is
    defined in selinux/selinux.h as
    
      typedef char *security_context_t;
    
    Signed-off-by: Eric Blake <eblake@redhat.com>