util/timespec: use unsigned 64 bit integers for nsec values

We added this utility for vulkan where all timeouts are given as uint64_t values. We can switch from signed to unsigned as this is the only user and if we ever deal with signed integers somewhere else we'll have to be careful to use the corresponding timespec_(add|sub)_msec and always pass absolute values.

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com Fixes: d2d70c3b ("util: add a timespec helper")