[eigen] Warnings with MKL and GCC

[Sam Webster]

Hi,

I'm getting enum-compare warnings when using MKL (2018.0) and GCC (5.4.0). It looks like there might be some int casts missing in Assign_MKL.h - https://github.com/samwebster/eigen/commit/46e6f1162d73073fe18712d8733b861ded0e9d25

Thanks,
Sam