MINIFICPP-2819 Fix LibSSH2 link failure on Windows

[lordgamez]

This change fixes the compilation issues of SFTP extension and tests, but SFTP test server is still not implemented on Windows so tests are still disabled on Windows for the time being, a new ticket is opened for fixing the tests: https://issues.apache.org/jira/browse/MINIFICPP-2820

https://issues.apache.org/jira/browse/MINIFICPP-2819

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