[c++] avrogencpp: emit deterministic include guard

[travisdowns]

What

CodeGen::guard() in lang/c++/impl/avrogencpp.cc suffixes the generated header's #ifndef guard with the output of an std::mt19937 seeded from ::time(nullptr):

string CodeGen::guard() {
    string h = headerFile_;
    makeCanonical(h, true);
    return h + "_" + std::to_string(random_()) + "_H";
}

So two invocations on the same schema produce headers whose guards differ:

#ifndef FOO_AVROGEN_H_3350718792_H
#ifndef FOO_AVROGEN_H_2362587291_H

This PR drops the random suffix.

Why

1. Generated output is non-deterministic. Re-running avrogencpp -i schema.avsc -o foo.h on the same input produces a different file every time, which is surprising for a codegen and makes side-by-side diff / git review difficult.

2. It breaks content-addressed build systems. Bazel's remote cache and the Nix store both key on input-content digests. With a randomised include guard, every consumer of the generated header sees a different input digest on every build — so on byte-identical schemas the entire downstream .cc → .o → .a → binary chain has to recompile and re-link, even though the schema hasn't changed. This was the trigger for filing the PR: a hermetic two-output-base Bazel build comparison flagged manifest_file.avrogen.h as a root non-hermetic action and traced ~hundreds of downstream cascade rebuilds back to it.

Why this is safe

headerFile_ is already guaranteed-unique per output (it's the path of the file we're about to write). makeCanonical(h, true) turns that path into a valid C identifier, which on its own is a fine guard name. The RNG suffix only added entropy, not uniqueness — there's no scenario where two avrogencpp runs producing headers at the same output path are supposed to coexist with conflicting guards.

After the change, the guard is <canonicalised-path>_H, deterministic across runs.

Test

Build avrogencpp, run it twice against the same schema (echo $RANDOM > /tmp/x.avsc is irrelevant — same schema, same path), check the outputs are byte-identical. Before the change they differ; after, they match.