Fix protocol buffer compiler warning

Fix protocol buffer compiler warning

Warning message:
  [ 86% 23128/26768] //hardware/ril/librilutils:librilutils protoc proto/sap-api.proto
  [libprotobuf WARNING external/protobuf/src/google/protobuf/compiler/parser.cc:547]
  No syntax specified for the proto file: hardware/ril/librilutils/proto/sap-api.proto.
  Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version.
  (Defaulted to proto2 syntax.)

Bug: 123039027
Test: build pass withtout warning message printed out
Change-Id: Ie340518a2f4758817bc3402fc897f918a8e89df6