(Re)activate SCTP system calls when compiling SCTP support into the kernel
r363079 introduced the possibility of loading the SCTP stack as a module in
addition to compiling it into the kernel. As part of this, the registration
of the system calls was removed and put into the loading of the module.
Therefore, the system calls are not registered anymore when compiling the
SCTP into the kernel. This patch addresses that.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D25632