babeld: Initial import, for Babel routing protocol.

* Initial import of the Babel routing protocol, ported to Quagga.
* LICENCE: Update the original LICENCE file to include all known potentially
  applicable copyright claims.  Ask that any future contributors to babeld/
  grant MIT/X11 licence to their work.
* *.{c,h}: Add GPL headers, in according with the SFLC guidance on
  dealing with potentially mixed GPL/other licensed work, at:

  https://www.softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.html
diff --git a/lib/thread.h b/lib/thread.h
index 69bb8d6..dfc51e2 100644
--- a/lib/thread.h
+++ b/lib/thread.h
@@ -23,6 +23,8 @@
 #ifndef _ZEBRA_THREAD_H
 #define _ZEBRA_THREAD_H
 
+#include <zebra.h>
+
 struct rusage_t
 {
 #ifdef HAVE_RUSAGE