Adds ifndef to headers

src/resolver/include/Resolver.hpp

@@ -1,3 +1,5 @@
+#ifndef RESOLVER_HPP
+#define RESOLVER_HPP
 #include "EventBus.hpp"
 
 class Resolver
@@ -5,3 +7,5 @@ class Resolver
     public:
     Resolver();
 };
+
+#endif