Add clauses to declared and defined.

Hi. Consider the following module:

``` haskell
data Dummy = D       

dummy : ()
dummy = ()
   where

instance Show Dummy where
  show _ = ""
```

Trying to load in repl:

```
*test> :r
Type checking ./test.idr
src/Idris/AbsSyntaxTree.hs:(372,1)-(381,50): Non-exhaustive patterns in function dec
lared                                                                              
```

The problem is empty __where__ clause. I just add clauses to _declared_ and _defined_. Is it right or not? Or maybe the problem in some uncatched parser failure? 
After the fix the code is working, though.