Import proto_library.gni when rtc_enable_protobuf is true

Import proto_library.gni when rtc_enable_protobuf is true instead of when
build_with_mozilla is false.

Makes it maybe easier to reason about the intention (e.g. intention is to not
compile any protobuf in, hence flag rtc_enable_protobuf)

The build file could not work if build_with_mozilla = true but
rtc_enable_protobuf = true.

Bug: webrtc:10338
Change-Id: I26e5983bd1519aa46c308b11796d518de5ef7597
Reviewed-on: https://webrtc-review.googlesource.com/c/123763
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26868}