ASACORE-1066: Add strdup() to Windows mapping.h

We started building with PolicyDB-enabled and our internal build system only
recognizes _strdup(), not strdup(). Adding this function to the Windows
mapping.h file fixes the issue.

Change-Id: I4350e2fb9f8d6539dadd88a0b295eee5bdabe763
Signed-off-by: Rob Smith <robs@microsoft.com>