ext/fast_mmaped_file_rs: add protobuf compilation step
requested to merge gitlab-community/ruby/gems/prometheus-client-mmap:add_protobuf_compilation_step into master
Breaking out protobuf compilation step from !132.
Tested by compiling and checking that
tmp/x86_64-linux/fast_mmaped_file_rs/3.1.2/target/release/build/fast_mmaped_file_rs-f298ac7ef311d558/out/io.prometheus.client.rs
exists.
Signed-off-by: Giedrius Statkevičius giedrius.statkevicius@vinted.com