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.

v2: Forgot to drop the test calling add_nsec() with a negative number

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reported-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Fixes: d2d70c3b ("util: add a timespec helper")
Acked-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 5833f433)