Add offline mirrors to .gitignore

Summary:
Add internal CocoaPods offline mirrors paths to .gitignore.

Changelog: [Internal]

Reviewed By: fbmal7

Differential Revision: D40241900

fbshipit-source-id: 712059f6d8f16bbd1964e13d49b257adad8aa8a2

.gitignore

@@ -106,6 +106,8 @@ package-lock.json
 # Ignore RNTester specific Pods, but keep the __offline_mirrors__ here.
 /packages/rn-tester/Pods/*
 !/packages/rn-tester/Pods/__offline_mirrors__
+!/packages/rn-tester/Pods/__offline_mirrors_hermes__
+!/packages/rn-tester/Pods/__offline_mirrors_jsc__
 
 # react-native-codegen
 /React/FBReactNativeSpec/FBReactNativeSpec