eal/windows: hide asprintf shim

eal/windows: hide asprintf shim

Make asprintf(3) implementation for Windows private to EAL, so that it's
hidden from external consumers. It is not exposed to internal consumers
either, because they don't need asprintf() and also because callers from
other modules would have no reliable way to free allocated memory.

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Khoa To <khot@microsoft.com>
Acked-by: Nick Connolly <nick.connolly@mayadata.io>
Acked-by: Ranjit Menon <ranjit.menon@intel.com>