For headers PKIX-SSH prefer to include other useful headers instead to duplicate declarations.
This is in contrarily to OpenBSD version where entropy.h is modified to duplicate declaration of sshbuf.

For instance removing inclusion of buffer header (entropy.h) indirectly removes include of some OpenSSL headers.
As result OpenSSL defines are lost and some headers will not detect properly OpenSSL features.
In other cases build fail due to missing OpenSSL structure declaration.
To prevent this lets in "includes.h to include OpenSSL type declarations and implicitly will be added OpenSSL types.

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