Fix unit test compilation error

A host command to set fan duty cycle is recently added and mock PWM
module doesn't provide the implementation. This breaks our unit test.
Let's fix this.

BUG=chrome-os-partner:10820
TEST='thermal' unit test passed.

Change-Id: I8644742cfec7d2112d7ff1e266b5ac3429c46945
Reviewed-on: https://gerrit.chromium.org/gerrit/26019
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>

chip/lm4/mock_pwm.c

@@ -54,6 +54,13 @@ int pwm_enable_keyboard_backlight(int enable)
 }
 
 
+int pwm_set_fan_duty(int percent)
+{
+	/* Not implemented */
+	return EC_SUCCESS;
+}
+
+
 void pwm_task(void)
 {
 	/* Do nothing */