Enable Xbios audio for Magic

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%403586

README.MiNT

@@ -76,7 +76,7 @@ Audio   O/S     Misc
 dma8    All     Uses MFP Timer A interrupt
 xbios	TOS     Uses MFP Timer A interrupt
 xbios   MiNT    Uses MFP Timer A interrupt
-xbios   Magic   Disabled
+xbios   Magic   Uses MFP Timer A interrupt
 stfa    All     Uses MFP interrupt
 mcsn	TOS     Uses MFP Timer A interrupt
 mcsn    MiNT    Uses MiNT thread