Make LSA buildable when one node is ACTIVE

refs: #2802

Change-Id: If3f99895522e22103130f32bb6634bf9a5691fe7
diff --git a/src/lsa.hpp b/src/lsa.hpp
index c797815..e149a6e 100644
--- a/src/lsa.hpp
+++ b/src/lsa.hpp
@@ -32,6 +32,8 @@
 
 namespace nlsr {
 
+class Nlsr;
+
 class Lsa
 {
 public: