images: add syntax specifier to autofs.proto

avoids:

[libprotobuf WARNING google/protobuf/compiler/parser.cc:547] No syntax specified for the proto file: autofs.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

images/autofs.proto

+syntax = "proto2";
+
 message autofs_entry {
 	required int32		fd			= 1;
 	required int32		pgrp			= 2;