netutils.h: define extern functions used by check_icmp.c

See,

check_icmp.c: In function ‘handle_random_icmp’:
check_icmp.c:460:18: warning: implicit declaration of function ‘address_length’ [-Wimplicit-function-declaration]
     char address[address_length(address_family)];
                  ^~~~~~~~~~~~~~
check_icmp.c:461:5: warning: implicit declaration of function ‘parse_address_string’; did you mean ‘parse_range_string’? [-Wimplicit-function-declaration]
     parse_address_string(address_family, addr, address, sizeof(address));
     ^~~~~~~~~~~~~~~~~~~~
     parse_range_string

plugins/netutils.h

@@ -84,6 +84,8 @@ void host_or_die(const char *str);
 extern int econn_refuse_state;
 extern int was_refused;
 extern int address_family;
+extern char address_length(int address_family);
+extern void parse_address_string(int address_family, struct sockaddr_storage *addr, char *address, int size);
 
 RETSIGTYPE socket_timeout_alarm_handler (int) __attribute__((noreturn));