altsvc: silence -Wsign-conversion

[vszakats]

altsvc.c: In function ‘altsvc_add’:
altsvc.c:192:18: warning: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Wsign-conversion]
  192 |       as->prio = prio;
      |                  ^~~~
altsvc.c: In function ‘getalnum’:
altsvc.c:412:9: warning: conversion to ‘size_t’ {aka ‘long unsigned int’} from ‘long int’ may change the sign of the result [-Wsign-conversion]
  412 |   len = p - protop;
      |         ^
altsvc.c: In function ‘altsvc_debugtime’:
altsvc.c:465:25: warning: conversion to ‘long unsigned int’ from ‘long int’ may change the sign of the result [-Wsign-conversion]
  465 |     unsigned long val = strtol(timestr, NULL, 10);
      |                         ^~~~~~
altsvc.c: In function ‘Curl_altsvc_parse’:
altsvc.c:549:19: warning: conversion to ‘size_t’ {aka ‘long unsigned int’} from ‘long int’ may change the sign of the result [-Wsign-conversion]
  549 |             len = p - hostp;
      |                   ^
altsvc.c:627:24: warning: conversion to ‘time_t’ {aka ‘long int’} from ‘long unsigned int’ may change the sign of the result [-Wsign-conversion]
  627 |               maxage = num;
      |                        ^~~
altsvc.c: In function ‘Curl_altsvc_lookup’:
altsvc.c:699:18: warning: conversion to ‘unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
  699 |        (versions & as->dst.alpnid)) {
      |                  ^

Ref: https://github.com/curl/curl/actions/runs/8819398779/job/24210519501#step:30:25

Closes #13466

[vszakats]

Closing in favour of #13481.