Add support for building voices with the swift engine from cepstral.com. Patch FS#7689 by Mario Lang.


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@14565 a1c6a512-1295-4272-9138-f99709370657
diff --git a/docs/CREDITS b/docs/CREDITS
index 19ea431..b08ab1a 100644
--- a/docs/CREDITS
+++ b/docs/CREDITS
@@ -321,6 +321,7 @@
 Boris Gjenero
 Sylvain Fourmanoit
 Alex Parker
+Mario Lang
 The libmad team
 The wavpack team
 The ffmpeg team