Update date in version output

COPYING

@@ -1,4 +1,4 @@
-Copyright 2011-2020 David Robillard <d@drobilla.net>
+Copyright 2011-2021 David Robillard <d@drobilla.net>
 
 Permission to use, copy, modify, and/or distribute this software for any
 purpose with or without fee is hereby granted, provided that the above

src/serdi.c

@@ -85,7 +85,7 @@ static int
 print_version(void)
 {
   printf("serdi " SERD_VERSION " <http://drobilla.net/software/serd>\n");
-  printf("Copyright 2011-2020 David Robillard <d@drobilla.net>.\n"
+  printf("Copyright 2011-2021 David Robillard <d@drobilla.net>.\n"
          "License: <http://www.opensource.org/licenses/isc>\n"
          "This is free software; you are free to change and redistribute it."
          "\nThere is NO WARRANTY, to the extent permitted by law.\n");