fix --strict-warnings build

C++ style comments are not allowed in ISO C90

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8693)

include/internal/refcount.h

diff --git a/include/internal/refcount.h b/include/internal/refcount.h
index f74f794..eddf124 100644
--- a/include/internal/refcount.h
+++ b/include/internal/refcount.h
@@ -106,7 +106,7 @@
 #     if _WIN32_WCE >= 0x600
        extern long __cdecl _InterlockedExchangeAdd(long volatile*, long);
 #     else
-       // under Windows CE we still have old-style Interlocked* functions
+       /* under Windows CE we still have old-style Interlocked* functions */
        extern long __cdecl InterlockedExchangeAdd(long volatile*, long);
 #      define _InterlockedExchangeAdd InterlockedExchangeAdd
 #     endif