mptcp: Allow compiling with MPTCP disabled

The static key is defined in mptcp_ctrl.c. So, it would not compile if
CONFIG_ENABLED is set to n. Thus, we add redefine mptcp() in case
CONFIG_MPTCP is disabled.

Fixes: 1ec7a15fac1f81d4 (mptcp: Add static-key check in mptcp())
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>