asound.h: include <time.h> to get struct timespec prototype

without including it, we get
In file included from mixer.c:44:0:
include/sound/asound.h:337:18: error: field 'trigger_tstamp' has incomplete type
include/sound/asound.h:338:18: error: field 'tstamp' has incomplete type
etc.
diff --git a/include/sound/asound.h b/include/sound/asound.h
index a041628..7c6de81 100644
--- a/include/sound/asound.h
+++ b/include/sound/asound.h
@@ -12,6 +12,7 @@
 #ifndef __SOUND_ASOUND_H
 #define __SOUND_ASOUND_H
 
+#include <time.h>
 #include <linux/types.h>
 
 #define SNDRV_PROTOCOL_VERSION(major, minor, subminor) (((major)<<16)|((minor)<<8)|(subminor))