Make exodiff find int64_t
Created by: jbakosi
This adds a standard library header so that int64_t (bottom of the file) can be found.
A little more info: I'm building a fully static build on Alpine Linux, which uses musl-libc instead of gnu-libc. As a pre-requisite, I supply my own BLAS and LAPACK (static libs) and the system-wide gcc (musl-gcc wrapper) needed this include to find int64_t.