lxc_monitor_protocol: Redefine xdr_uint64_t if needed

lxc_monitor_protocol: Redefine xdr_uint64_t if needed

https://bugzilla.redhat.com/show_bug.cgi?id=993411

On some systems (using libtirpc instead of glibc's
implementation), xdr_uint64_t exists rather under different name:
xdr_u_int64_t. This makes compilation fail then:

libvirt_lxc-lxc_monitor_protocol.o: In function `xdr_virLXCMonitorInitEventMsg':
/usr/local/src/libvirt/libvirt-1.1.1/src/./lxc/lxc_monitor_protocol.c:31: undefined reference to `xdr_uint64_t'

Therefore we rather mirror the d707c866 commit and redefine
xdr_uint64_t if needed.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>