mk: Add rtstartup to dist

[alexcrichton]

Needed for distcheck to pass and to have a working tarball.

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[nikomatsakis]

@bors r+

Seems harmless enough.

[bors]

📌 Commit 6e27448 has been approved by nikomatsakis

[bors]

⌛ Testing commit 6e27448 with merge 8fa8684...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt

[Manishearth]

Windows still broken http://buildbot.rust-lang.org/builders/nightly-dist-rustc-win-msvc-64/builds/152/steps/distcheck/logs/stdio

ls: cannot access x86_64-pc-windows-msvc/stage2/lib/std-*.dll: No such file or directory
ls: cannot access x86_64-pc-windows-msvc/stage2/lib/std-*.lib: No such file or directory
install: missing destination file operand after 'tmp/dist/rustc-nightly-x86_64-pc-windows-msvc-image/lib/'
Try 'install --help' for more information.
make: *** [prepare-host-lib-std-2-x86_64-pc-windows-msvc-dir-x86_64-pc-windows-msvc] Error 1