Fix trailing slashes in the Servo makefile.

This fixes a build issue where GNU Make 4.0 would stop with:

    No rule to make target '…/rust-http/src/http/generated/read_method.rs',
    needed by 'libhttp.dummy'.  Stop.

Apparently other people did not have this issue,
it probably only affects some Make versions.
Merge pull request #15 from SimonSapin/makefile-trailing-slashes

Fix trailing slashes in the Servo makefile.