What’s happening is that Eigen is using the optimized vector processing provided by the Neon processor that the V5 has, that’s part of a system header file, arm_neon.h. There’s a mismatch between what that header thinks is an int32_t and what the VEX sdk thinks is an int32_t, it may be a clang/gcc difference, not sure. Anyway, for now turn off Neon detection like this and it will build.
#include "vex.h"
#include <iostream>
#undef __ARM_NEON__
#undef __ARM_NEON
#include "Eigen/Dense"