Add missing backend dev-dependency

[jturner314]

Since the default feature set of ndarray-linalg is empty, examples and tests were unable to build since no backend was selected. This commit adds a dev-dependency to use the OpenBLAS backend.

Steps to reproduce the issue that this PR fixes:

$ git clone https://github.com/termoshtt/ndarray-linalg.git
$ cd ndarray-linalg
$ cargo test
...
error: linking with `cc` failed: exit code: 1
...
error: Could not compile `ndarray-linalg`.
...

I'm surprised this hasn't come up earlier. Are other people able to run cargo test without this PR?

[jturner314]

Oh, I see. CI is set up to pass the backed with the --features command line argument.