Adds the podspec to objectivec_EXTRA_DIST in Makefile.am

Makefile.am

@@ -318,7 +318,8 @@ objectivec_EXTRA_DIST=                                                       \
   objectivec/Tests/unittest_runtime_proto2.proto                              \
   objectivec/Tests/unittest_runtime_proto3.proto                              \
   objectivec/Tests/UnitTests-Bridging-Header.h                                \
-  objectivec/Tests/UnitTests-Info.plist
+  objectivec/Tests/UnitTests-Info.plist                                       \
+  Protobuf.podspec
 
 python_EXTRA_DIST=                                                           \
   python/google/protobuf/internal/api_implementation.cc                      \