Fix bug introduced in cdbe73eb47

Apparently I don't understand how pattern-matching works in Haskell

src/compiler/GF/Compile/Compute/Predef.hs

@@ -28,8 +28,8 @@ instance Predef Int where
 instance Predef Bool where
   toValue = boolV
   fromValue v = case v of
-    VCApp (cPredef,cPTrue) [] -> return True
-    VCApp (cPredef,cPFalse) [] -> return False
+    VCApp (mn,i) [] | mn == cPredef && i == cPTrue -> return True
+    VCApp (mn,i) [] | mn == cPredef && i == cPFalse -> return False
     _ -> verror "Bool" v
 
 instance Predef String where