included only necessary headers

If users want to use only pcm.c or mixer.c
they can. It's not necessary to include
tinyalsa/asoundlib.h because mixer.c doesn't
use pcm.h and pcm.c doesn't use mixer.h.
diff --git a/src/mixer.c b/src/mixer.c
index cf728e7..7f271e4 100644
--- a/src/mixer.c
+++ b/src/mixer.c
@@ -45,7 +45,7 @@
 #define __user
 #include <sound/asound.h>
 
-#include <tinyalsa/asoundlib.h>
+#include <tinyalsa/mixer.h>
 
 /** A mixer control.
  * @ingroup tinyalsa-mixer