ORB.cl : fixed redefinition of val

When setting WTA_K to 4, int val is redefined leading to a console error output and thus slowing the processing down massively (roughly 2 frames per second).

FYI, this is the console output at runtime:
```
OpenCL program build log: -D ORB_DESCRIPTORS -D WTA_K=4
:156:24: error: redefinition of 'val'
int t0, t1, t2, t3, k, val;
                       ^
:124:5: note: previous definition is here
int val;
```