Use google.api instead of custom googleapi package

Use the standard google.api package to not create conflicts with other
proto definition that uses the same package.

Change-Id: I264db977a2316ced0fdfb6ab8e499a1258726cb7
diff --git a/protoc_plugins/swagger_gen.py b/protoc_plugins/swagger_gen.py
index 3b558d6..67d6a9a 100755
--- a/protoc_plugins/swagger_gen.py
+++ b/protoc_plugins/swagger_gen.py
@@ -20,7 +20,7 @@
 from simplejson import dumps
 
 # without this import, http method annotations would not be recognized:
-from chameleon.protos import annotations_pb2, http_pb2
+from google.api import annotations_pb2, http_pb2
 
 from chameleon.protoc_plugins.descriptor_parser import DescriptorParser
 from swagger_template import native_descriptors_to_swagger