Adds watch os deployment target for protobuf

Protobuf.podspec

@@ -36,5 +36,6 @@ Pod::Spec.new do |s|
 
   s.ios.deployment_target = '7.1'
   s.osx.deployment_target = '10.9'
+  s.watchos.deployment_target = '2.0'
   s.requires_arc = false
 end