follow-up to earlier music.c change

After a music.c change earlier today, an annoying compiler warning resulted.
.\music.c(721): warning C4295: 'notes': array is too small to include
a terminating null character

src/music.c

@@ -718,7 +718,7 @@ do_improvisation(struct obj* instr)
 static char *
 improvised_notes(boolean *same_as_last_time)
 {
-    static const char notes[7] = "ABCDEFG"; /* 7: no trailing '\0' */
+    static char notes[7] = { 'A', 'B', 'C', 'D', 'E', 'F', 'G' };
     /* target buffer has to be in gc.context, otherwise saving game 
      * between improvised recitals would not be able to maintain
      * the same_as_last_time context. */