knownhost.c: use LIBSSH2_FREE macro instead of free
knownhost.c: use LIBSSH2_FREE macro instead of free

Use LIBSSH2_FREE instead of free since
_libssh2_base64_encode uses LIBSSH2_ALLOC

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2