C200: Also flip the function of the volume buttons when display flip is enabled. I think it is more consistent, especially when thinking of cases where these buttons are used for something else (e.g. left/right in brickmania). Taken from FS#8393 by Volker Mische - add him to CREDITS too.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@18404 a1c6a512-1295-4272-9138-f99709370657
diff --git a/docs/CREDITS b/docs/CREDITS
index dff8826..391edec 100644
--- a/docs/CREDITS
+++ b/docs/CREDITS
@@ -415,6 +415,7 @@
 Tadeusz Pyś
 Rostislav Chekan
 Florin Popescu
+Volker Mische
 
 The libmad team
 The wavpack team