polarssl: enable AES-GCM and CAMELLIA-GCM ciphersuites

Recent versions of Chrome require this ciphers to successfully handshake with
a TLS enabled uhttpd server using the ustream-polarssl backend.

If `CONFIG_GCM` is disabled, `ssl_ciphersuite_from_id()` will return `NULL`
when cipher `0x9d` is looked up, causing the calling `ssl_ciphersuite_match()`
to fail with `POLARSSL_ERR_SSL_INTERNAL_ERROR`.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>