Fixed breakage from removing the used include of <sys/buf.h> in the

previous commit:
Include <sys/mutex.h> and its prerequisite <sys/lock.h> instead of
depending on namespace pollution in <sys/buf.h>.  Only do this in the
__FreeBSD_version > 500000 case although the __FreeBSD_version < 500000
case has already rotted.

sys/dev/sound/pcm/sound.h

@@ -73,6 +73,9 @@
 #undef	USING_DEVFS
 
 #if __FreeBSD_version > 500000
+#include <sys/lock.h>
+#include <sys/mutex.h>
+
 #define USING_MUTEX
 #define USING_DEVFS
 #endif