Update GoogleAnalytics-iOS-SDK.podspec

Update to include header files for googleTagManager, by adding folder to source_files.

reason: `libGoogleAnalyticsServices.a` is now a combined SDK containing both google analytics and google tag manager. (See `readme` in the SDK [GoogleAnalyticsServicesiOS_3.01.zip](http://dl.google.com/googleanalyticsservices/GoogleAnalyticsServicesiOS_3.01.zip)