Subject: [PATCH] Adds "REQUIRES PRIVATE KEY" callback

In some situations the private key is not directly available via
filesystem (for example, when stored on a smartcard). If this is
the case, the user can set the private key filepath to NULL and
expect this callback reason to set the key directly via openSSL
library calls.
fix #220. Add missing include statement for <cstdarg>
https://github.com/warmcat/libwebsockets/commit/6ff28248aadc12b971d13b212587e63b49a4653f#commitcomment-8861797

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