song_save: pass const songvec to songvec_save()

src/song_save.c

@@ -57,7 +57,7 @@ song_save(struct song *song, void *data)
 	return 0;
 }
 
-void songvec_save(FILE *fp, struct songvec *sv)
+void songvec_save(FILE *fp, const struct songvec *sv)
 {
 	songvec_for_each(sv, song_save, fp);
 }

src/song_save.h

@@ -29,7 +29,8 @@
 struct songvec;
 struct directory;
 
-void songvec_save(FILE *fp, struct songvec *sv);
+void
+songvec_save(FILE *fp, const struct songvec *sv);
 
 /**
  * Loads a song from the input file.  Reading stops after the