Move dirs reallocation from dirpool_make_dirtraverse to dirpool_add_dir

Move dirs reallocation from dirpool_make_dirtraverse to dirpool_add_dir

repo_add_solv does not use the correct DIR_BLOCK allocation, so we
need to extend the dirs array the first time we add a new directory.

We used to do this in dirpool_make_dirtraverse, but since commit
fa35e8672b478c410429d403b887277d7b17e57d the dirtraverse array is
no longer created when calling dirpool_add_dir. So we now do it
the first time the hash is created.