Adjust sctp_drain() definition to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

sys/netinet/sctp_pcb.c:6946:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
sctp_drain()
          ^
           void

This is because sctp_drain() is declared with a (void) argument list,
but defined with an empty argument list. Make the definition match the
declaration.

MFC after: 3 days