selftests: ALSA: fix memory leak in utimer test

Free the malloc'd buffer in TEST_F(timer_f, utimer) to prevent
memory leak.

Fixes: 1026392d10af ("selftests: ALSA: Cover userspace-driven timers with test")
Reported-by: Jun Zhan <[e-mail]>
Signed-off-by: WangYuli <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

tools/testing/selftests/alsa/utimer-test.c

···
 	pthread_join(ticking_thread, NULL);
 	ASSERT_EQ(total_ticks, TICKS_COUNT);
 	pclose(rfp);
+	free(buf);
 }
 
 TEST(wrong_timers_test) {