net/44bsd-rdist: Permanently fix LLVM15 errors

Fix:

expand.c:182:19: error: incompatible pointer to integer conversion
passing 'void *' to parameter of type 'int' [-Wint-conversion]
                tp = lookup(cp, NULL, 0);
                                ^~~~
/usr/include/sys/_null.h:34:14: note: expanded from macro 'NULL'
                ^~~~~~~~~~~