Add mpc-1.3.1 file

[GuillaumeGomez]

Needed for rust-lang/rust#148481:

  mpc-1.3.1.tar.gz: FAILED
  sha512sum: WARNING: 1 computed checksum did NOT match
  error: Cannot verify integrity of possibly corrupted file mpc-1.3.1.tar.gz
  Command `/checkout/obj/build/x86_64-unknown-linux-gnu/gcc/src/contrib/download_prerequisites [workdir=/checkout/obj/build/x86_64-unknown-linux-gnu/gcc/src]` failed with exit code 1

Tomorrow I'll check all files to be downloaded so we can escape the "add new file/restart CI" cycle.

cc @marcoieni