修正 "Flutter/Flutter.h not found." on iOS. 的问题
参考：https://github.com/flutter/flutter/issues/16036#issuecomment-380712094
iOS环境：

[] iOS toolchain - develop for iOS devices (Xcode 9.3.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 9.3.1, Build version 9E501
• ios-deploy 1.9.2
• CocoaPods version 1.5.2