[nrf fromtree] lib: os: clock: Fix possibly unitialized variable warning

When building with high optimization level, the compiler thinks
duration may be used initialized and warns as much.

Let's initialize this variable always to ensure it does not happen
and with it pacify the compiler.

Signed-off-by: Alberto Escolar Piedras <[email protected]>
(cherry picked from commit 1d6da40)

Author: aescolar

lib/os/clock.c

@@ -158,7 +158,7 @@ int z_impl_sys_clock_nanosleep(int clock_id, int flags, const struct timespec *r
 {
 	k_timepoint_t end;
 	k_timeout_t timeout;
-	struct timespec duration;
+	struct timespec duration = {0, 0};
 	const bool update_rmtp = rmtp != NULL;
 	const bool abstime = (flags & SYS_TIMER_ABSTIME) != 0;