timeval.c: Use long long constant type for timeval assignment

On a 64 bit host, sparse says:

timeval.c:148:15: warning: constant 0x7fffffffffffffff is so big it is long
timeval.c:149:12: warning: constant 0x7fffffffffffffff is so big it is long

so let's use long long constant types in order to prevent undesired overflow
failures.

Bug: https://curl.haxx.se/mail/lib-2017-07/0003.html



Closes #1636

Signed-off-by: Martin Kepplinger <martink@posteo.de>