[mlir] Add header file for ssize_t

ssize_t is part of POSIX and not standard C/C++, so using ssize_t
without the necessary header files causes the build to fail on Windows
with the following error: 'ssize_t': undeclared identifier.

This patch includes llvm/Support/DataTypes.h to resolve the problem.

Differential Revision: https://reviews.llvm.org/D141149