Fix MINIMAL build

Pre-declare struct ucred, to fix build issues on the MINIMAL config:

In file included from /usr/src/sys/netpfil/pf/pfsync_nv.c:40:
/usr/src/sys/netinet6/ip6_var.h:384:31: error: declaration of 'struct ucred' will not be visible outside of this function [-Werror,-Wvisibility]

struct ip6_pktopts *, struct ucred *, int);
                             ^

/usr/src/sys/netinet6/ip6_var.h:408:28: error: declaration of 'struct ucred' will not be visible outside of this function [-Werror,-Wvisibility]

struct inpcb *, struct ucred *, int, struct in6_addr *, int *);
                       ^

2 errors generated.