m/p/api: use protobuf.Timestamp in LogEntry

This updates LogEntry to use google.protobuf.Timestamp.
See: issue #129.

Change-Id: I937800aa91e86690da0d06f743e720c2d474ad0a
Reviewed-on: https://review.monogon.dev/c/monogon/+/832
Tested-by: Jenkins CI
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/pkg/logtree/BUILD.bazel b/metropolis/pkg/logtree/BUILD.bazel
index 0df5679..aabc40d 100644
--- a/metropolis/pkg/logtree/BUILD.bazel
+++ b/metropolis/pkg/logtree/BUILD.bazel
@@ -21,6 +21,7 @@
     deps = [
         "//metropolis/pkg/logbuffer",
         "//metropolis/proto/api",
+        "@org_golang_google_protobuf//types/known/timestamppb",
     ],
 )