Move the definition of SCTP's system_base_info into sctp_crc32.c.

This file is the only SCTP source file compiled into the kernel when
SCTP_SUPPORT is configured. sctp_delayed_checksum() references a couple
of counters defined in system_base_info, so the change allows these
counters to be referenced in a kernel compiled without "options SCTP".

Submitted by: tuexen
MFC with: r362338