Add missing proto syntax declaration

This silences a warning that the proto syntax version is unspecified
during compilation for keystore_client.proto.  The compiler assumes
proto2 if it's unspecified so this is a noop for the generated code.

Bug: None
Test: None
Change-Id: Ib3703be89e661c409156b148b6d041118606bd06
diff --git a/keystore/keystore_client.proto b/keystore/keystore_client.proto
index cd520dc..cbafd54 100644
--- a/keystore/keystore_client.proto
+++ b/keystore/keystore_client.proto
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+syntax = "proto2";
+
 package keystore;
 
 option optimize_for = LITE_RUNTIME;