exp/audio: add instruction for installing required package

The error message from cgo will print the line, which now
includes the instruction as a comment.

Change-Id: I208365b5b1e4da0bd6df89882586fe438f7391b4
Reviewed-on: https://go-review.googlesource.com/25384
Reviewed-by: David Crawshaw <crawshaw@golang.org>

exp/audio/al/al_notandroid.go

@@ -19,7 +19,7 @@ package al
 
 #ifdef GOOS_linux
 #include <stdlib.h>
-#include <AL/al.h>
+#include <AL/al.h>  // install on Ubuntu with: sudo apt-get install libopenal-dev
 #endif
 */
 import "C"