make: update dist target for renamed files

src/README.txt has been renamed in a30d61b05a, make dist is
thus failing as unable to find it.
This commit makes the dist target working again.

Signed-off-by: Luca Bruno <lucab@debian.org>
make: update dist target after libextra split

libextra is currently being split into several crates. This commit adds
them all to the dist target in order to have them in the final tarballs.

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #12112 : lucab/rust/make-dist, r=alexcrichton

`make dist` or building from a generated tarball is currently not possible due to some files that have been renamed and the ongoing libextra split. This PR fixes all (current) issues in order to build rust from the .tar.gz source alone.