Add missing load statement for proto_library

[adincebic]

Generated BUILD.bazel file does not contain required load statement for proto_library:

load("@rules_proto//proto:defs.bzl", "proto_library")

which results in following error on bazel 8:

ERROR: /private/var/tmp/_bazel_adincebic/1899d14f6acb23e5b475897f501caf4b/external/toolchains_protoc++protoc+toolchains_protoc_hub.osx_aarch_64/BUILD.bazel:13:1: name 'proto_library' is not defined

Not sure if this happens on earlier versions of bazel as this rule might have been vendored similar to rules_android and others.

[CLAassistant]

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[alexeagle]

Thanks - I suspect we'll have to give up on this feature of creating libraries for the well-known-types but you're right this is better.