wraps libssh2_session_flag method

libssh2_session_flag is required in order to activate compression.

Allows fixing issue #7:

    $ssh2 = Net::SSH2->new;
    $ssh2->flag(LIBSSH2_FLAG_COMPRESS, 1);
    $ssh2->connect($server, 22) or die $!;
    print 'COMP_SC: ' . $ssh2->method('COMP_SC') . ' COMP_CS: ' .$ssh2->method('COMP_CS');