Fix error compiling AsmCodeGen.lhs for PPC Mac (DestBlockId)

The error message eliminated is:
> compiler/nativeGen/AsmCodeGen.lhs:637:16:
>     Not in scope: data constructor `DestBlockId'

compiler/nativeGen/PPC/RegInfo.hs

@@ -7,7 +7,7 @@
 -----------------------------------------------------------------------------
 
 module PPC.RegInfo (
-        JumpDest, 
+        JumpDest( DestBlockId ), 
 	canShortcut, 
 	shortcutJump,