Fix redundant import errors in nalgebra-glm

[yotamofek]

More fallout from rust-lang/rust#121708
Should make CI green again

[Andlon]

Thanks for figuring out the cause of this @yotamofek! I'm unsure why this compiles though. There's a call to na::zero on Line 7. How does this compile without na having been explicitly imported? What am I missing?

[Andlon]

Ah, I see; it's because of extern crate nalgebra as na; in lib.rs. OK, I guess it should be safe to merge as far as I can tell.

@tpdickso: you've been looking into this issue. Does this adequately fix the issue or can you think of any lingering problems?

[tpdickso]

@Andlon The top-level crate has extern nalgebra as na; which is sufficient to put it in the namespace, and is what made the use na; in the file redundant in the first place.

Anyway, this is good to merge! I don't expect it to fix the other redundant import issues which were being caused by an "invisible" use na; on line 2 of several other files -- but those seem to have resolved themselves, perhaps with a version bump of xargo or something.