cobalt/kernel: Null-terminate cobalt_print_sched_params output

Using trace_seq_printf requires to terminate the generated string with
0, or we print additional garbage.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kernel/cobalt/trace/cobalt-posix.h

@@ -129,6 +129,7 @@ DECLARE_EVENT_CLASS(syscall_exit,
 				 (__p_ex)->sched_priority);		\
 		break;							\
 	}								\
+	trace_seq_putc(p, '\0');					\
 	__ret;								\
 })