mptcp: mptcp_parse_option is no longer exported

Options parsing in now done from mptcp_incoming_options().

mptcp_parse_option() has been removed from mptcp.h when CONFIG_MPTCP is
defined but not when it is not.

Fixes: cfde141e ("mptcp: move option parsing into mptcp_incoming_options()")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>