Add ly version t0 swing.ly (issue 562850043 by address@hidden)

[thomasmorley65]

Reviewers: ,

Message:
While testing GUB this one popped up.
Not sure if it's the cause of GUB-fail, but should be fixed anyway.
As soon as the automated tests succeed I'll push it prematurely.

Description:
Add ly version t0 swing.ly

Please review this at https://codereview.appspot.com/562850043/

Affected files (+1, -0 lines):
  M ly/swing.ly


Index: ly/swing.ly
diff --git a/ly/swing.ly b/ly/swing.ly
index  
f3349d0463a390471ccb60ce5950a806e41bd806..478b579819a7d03372d1998c83fb561606d54978  
100644
--- a/ly/swing.ly
+++ b/ly/swing.ly
@@ -91,6 +91,7 @@
 %%
 %%   will result in timing corresponding to \times 2/3 { c4 c16 c }.

+\version "2.21.0"

 %%% Helper functions