kernel: usage: fix unused-variable warning when THREAD_USAGE_ALL=n

With CONFIG_SCHED_THREAD_USAGE_ANALYSIS=y and
CONFIG_SCHED_THREAD_USAGE_ALL=n, sched_cpu_update_usage() is a no-op
macro, so the cycles local in k_thread_runtime_stats_enable() and
z_thread_stats_reset() is set but never read, triggering a compiler
warning. Pass the expression straight to the macro to keep
that configuration warning-free.

Signed-off-by: Yiren Guo <guoyr_2013@hotmail.com>

kernel/usage.c

diff --git a/kernel/usage.c b/kernel/usage.c
index 750f1ab..86fcdeb 100644
--- a/kernel/usage.c
+++ b/kernel/usage.c
@@ -260,9 +260,8 @@
 
 		if (thread == cpu->current) {
 			uint32_t now = usage_now();
-			uint32_t cycles = now - cpu->usage0;
 
-			sched_cpu_update_usage(cpu, cycles);
+			sched_cpu_update_usage(cpu, now - cpu->usage0);
 
 			cpu->usage0 = now;
 		}
@@ -436,9 +435,8 @@
 	/* Update the current CPU stats. */
 
 	uint32_t now = usage_now();
-	uint32_t cycles = now - _current_cpu->usage0;
 
-	sched_cpu_update_usage(_current_cpu, cycles);
+	sched_cpu_update_usage(_current_cpu, now - _current_cpu->usage0);
 
 	_current_cpu->usage0 = now;