modules: civetweb: Add proper dependency to Kconfig

Make sure that CONFIG_NET_TCP_ISN_RFC6528 is not set when compiling
Civetweb. There are compile issues in Civetweb if both mbedtls and
POSIX API option are set, and this happens if the TCP ISN option is
enabled.

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