Added missing parenthesis in C code generation.

The following code fails to compile:

```
impure_op : Bits64 -> IO Bits64
impure_op foo = return $ foo + 1

impure_int : IO Int
impure_int = return 41

main : IO ()
main = impure_int >>= impure_op . prim__zextInt_B64 >>= print
```

The error message is:
```
/tmp/idris5151: In function ‘_idris_prim_95__95_zextInt_95_B64’:
/tmp/idris5151:615:56: error: expected ‘)’ before ‘;’ token
     RVAL = idris_b64const(vm, (uintptr_t)GETINT(LOC(0));
                                                        ^
```