proto: Change pcrs to uint32
proto: Change pcrs to uint32

This makes our file more consistent as we use uint32 for PCR indicies
elsewhere. This is compatible as int32 and uint32 have the same
representation:

https://developers.google.com/protocol-buffers/docs/proto3#updating

google/go-tpm-tools,google/go-tpm-tools,google/go-tpm-tools