virnuma: Implement virNumaGetDistances stub for non-NUMA

virnuma: Implement virNumaGetDistances stub for non-NUMA

In case the libvirt is built without numactl support, we're
missing the virNumaGetDistances() stub so the linking fails:

  CCLD     libvirt_lxc
libvirt_lxc-nodeinfo.o: In function `virNodeCapsGetSiblingInfo':
/home/zippy/tmp/libvirt.git/src/nodeinfo.c:1763: undefined reference to `virNumaGetDistances'
collect2: error: ld returned 1 exit status
make[3]: *** [libvirt_lxc] Error 1

The issue was introduced in 77c830d8c4.

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