gRPC-Core.podspec: init submodules, for nanopb
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
index 54e652e..5d21ca9 100644
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@@ -46,6 +46,7 @@
   s.source = {
     :git => 'https://github.com/grpc/grpc.git',
     :tag => "release-#{version.gsub(/\./, '_')}-objectivec-#{version}",
+    :submodules => true,
   }
 
   s.ios.deployment_target = '7.1'