Add missing XDR_FLAGS

Add missing XDR_FLAGS

Fixes build problems on x86_64-cygwin host for aarch64 target:
  CC       lxc/libvirt_driver_lxc_impl_la-lxc_monitor_protocol.lo
In file included from lxc/lxc_monitor_protocol.c:7:0:
lxc/lxc_monitor_protocol.h:9:21: fatal error: rpc/rpc.h: No such file or directory

  CC       rpc/libvirt_setuid_rpc_client_la-virnetmessage.lo
In file included from rpc/virnetmessage.h:24:0,
                 from rpc/virnetmessage.c:26:
rpc/virnetprotocol.h:9:21: fatal error: rpc/rpc.h: No such file or directory

  CC       lxc/libvirt_lxc-lxc_monitor_protocol.o
In file included from lxc/lxc_monitor_protocol.c:7:0:
lxc/lxc_monitor_protocol.h:9:21: fatal error: rpc/rpc.h: No such file or directory

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>