virnetlink: fix build on non-Linux
    
    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: Eric Blake <eblake@redhat.com>