PFLOG: Try to fix previous commit

PFLOG: Try to fix previous commit

The error was:
./print-pflog.c:99:20: error: passing 'char const[16]' to parameter
of type 'const u_char *' (aka 'const unsigned char *') converts
between pointers to integer types with different sign
[-Werror,-Wpointer-sign]
                nd_printjnp(ndo, hdr->ruleset, PFLOG_RULESET_NAME_SIZE);
                                 ^~~~~~~~~~~~
./netdissect.h:397:61: note: passing argument to parameter here
extern void nd_printjnp(netdissect_options *, const u_char *, u_int);
                                                            ^