timeval: typecast for better type (on Amiga)

There is an issue with conflicting "struct timeval" definitions with
certain AmigaOS releases and C libraries, depending on what gets
included when.  It's a minor difference - the OS one is unsigned,
whereas the common structure has signed elements.  If the OS one ends up
getting defined, this causes a timing calculation error in curl.

It's easy enough to resolve this at the curl end, by casting the
potentially errorneous calculation to a signed long.