types: make flow happy

src/core/vdom/create-component.js

@@ -235,8 +235,9 @@ function installComponentHooks (data: VNodeData) {
   }
 }
 
-function mergeHook (f1, f2) {
+function mergeHook (f1: any, f2: any): Function {
   const merged = (a, b) => {
+    // flow complains about extra args which is why we use any
     f1(a, b)
     f2(a, b)
   }