samples: net: civetweb: Avoid using mbedtls

Civetweb does not work properly with mbedtls if Posix APIs
are enabled. For example time function prototypes are not found
by the mbedtls module if we enable mbedtls in the sample.

Disable TCP ISN RFC6528 support because of this for civetweb
samples, as setting that option will pull in mbedtls and
we get a compilation warning because time() is not declared
by any header file that mbedtls is including.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>