Add callback multiplex support

Change-Id: Iddeb3a1b0e20706e72ec8f74dabc60b342f003ba
Signed-off-by: Dave Barach <dave@barachs.net>
diff --git a/src/vlib/vlib.h b/src/vlib/vlib.h
index b5fe47b..8f59cae 100644
--- a/src/vlib/vlib.h
+++ b/src/vlib/vlib.h
@@ -42,6 +42,7 @@
 
 #include <vppinfra/clib.h>
 #include <vppinfra/elf_clib.h>
+#include <vppinfra/callback.h>
 
 /* Generic definitions. */
 #include <vlib/defs.h>