Fix a C++ comment in the refcount.h

Although in a false-conditional code section gcc-4.8.4 flagged this with
a C90 warning :-(

include/internal/refcount.h:108:7: error: C++ style comments are not allowed in ISO C90 [-Werror]
       // under Windows CE we still have old-style Interlocked* functions

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9388)