Set fishhook's header_dir to "fishhook"

Summary:
This lets us import fishhook like `<fishhook/fishhook.h>`.
Closes https://github.com/facebook/react-native/pull/16192

Differential Revision: D5971639

Pulled By: shergin

fbshipit-source-id: c46503953a08a74bdd10ed35cbd30029f1666c7d

React.podspec

@@ -204,6 +204,7 @@ Pod::Spec.new do |s|
   end
 
   s.subspec "fishhook" do |ss|
+    ss.header_dir           = "fishhook"
     ss.source_files         = "Libraries/fishhook/*.{h,c}"
   end