Fix hscolour style sheet building

Currently we only symlink this into two hard-coded directories, which
breaks the source code viewing for all the others.

The patch replaces this with automatic building; as playing with
relative symlinks is non-trivial, I just generate it many times (the
file is small enough (<250) that it shouldn't matter, size-wise, and
it takes 2ms to do it).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>