formdata: fix warning: 'CURLformoption' is promoted to 'int'

formdata: fix warning: 'CURLformoption' is promoted to 'int'

curl/lib/formdata.c: In function 'FormAdd':
curl/lib/formdata.c:249:31: warning: 'CURLformoption' is promoted to 'int' when passed through '...'
  249 |       option = va_arg(params, CURLformoption);
      |                               ^
curl/lib/formdata.c:249:31: note: (so you should pass 'int' not 'CURLformoption' to 'va_arg')
curl/lib/formdata.c:249:31: note: if this code is reached, the program will abort

Closes #9484