Add missing FindNode prototype to net.h

Also make the argument a const std::string & instead of pass-by-value.

src/net.cpp

@@ -456,7 +456,7 @@ CNode* FindNode(const CNetAddr& ip)
     return NULL;
 }
 
-CNode* FindNode(std::string addrName)
+CNode* FindNode(const std::string& addrName)
 {
     LOCK(cs_vNodes);
     BOOST_FOREACH(CNode* pnode, vNodes)