proto: Fix warnings for protoc v3

proto: Fix warnings for protoc v3

protoc v3 introduces a new syntax for proto files and warns when the
syntax is not explicitly stated.

protoc relies on the fact that undefined preprocessor symbols are
explanded to 0 but since we use -Wundef they end up generating
warnings.

Change-Id: If07abeb54e932469c8f2c4d38634a97fdae40f77
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>