Commit 4dc04d3a added virNetlinkGetErrorCode, but forgot to provide
a fallback, which kills the build on mingw (among others):

  CCLD     libvirt.la
  Cannot export virNetlinkGetErrorCode: symbol not defined
  collect2: error: ld returned 1 exit status

* src/util/virnetlink.c (virNetlinkGetErrorCode): Provide fallback.
Signed-off-by: NEric Blake <eblake@redhat.com>