Explicitly add python-numpy dependency to install-build-deps.
The package python-opencv recently added to install-build-deps depends on the python-numpy package. However, on Ubuntu 12.*, the package manager doesn't know about this dependency, so python-opencv crashes without explicitly installing python-numpy.
So basically, this just explicitly lists a dependency that already was a dependency.
BUG=424345
Review URL: https://codereview.chromium.org/
744883002
Cr-Commit-Position: refs/heads/master@{#305214}