gen.go: explicitly list top-level .proto files
gen.go: explicitly list top-level .proto files

This forces a consistent order of processing, which is required
because protoc-gen-go puts extra comments in the first file
processed and we need this to always be the same file.

google/trillian,daviddrysdale/trillian,codingllama/trillian,RJPercival/trillian,daviddrysdale/trillian,RJPercival/trillian,daviddrysdale/trillian,Martin2112/trillian,gtank/trillian,gdbelvin/trillian,codingllama/trillian,gdbelvin/trillian,AlCutter/trillian,google/trillian,gtank/trillian,Martin2112/trillian,Martin2112/trillian,AlCutter/trillian,gdbelvin/trillian