[Core] Link WatchKit for watchOS (#10157)

Author: ncooke3

FirebaseCore.podspec

@@ -45,6 +45,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
   s.ios.framework = 'UIKit'
   s.osx.framework = 'AppKit'
   s.tvos.framework = 'UIKit'
+  s.watchos.framework = 'WatchKit'
 
   # Remember to also update version in `cmake/external/GoogleUtilities.cmake`
   s.dependency 'GoogleUtilities/Environment', '~> 7.7'