Add mmap2 to deps.rs.

[ltratt]

Can we get rid of mmap at the same time?

[vext01]

A couple of places in rustc still use the old mmap. This should be fixed upstream though.

(We currently don't run security audits on ykrustc, as upstream code always upsets cargo audit)

[ltratt]

OK.

bors r+

[bors[bot]]

Build succeeded:

• buildbot/yorick-buildbot-build-script-hw