Fix strdup macro redefinition

This fixes the following error when the CRT debug heap (crtdbg.h) is used:
e_os.h(476): warning C4005: 'strdup': macro redefinition
C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\ucrt\crtdbg.h(319): note: see previous definition of 'strdup'
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1137)