Fix implicit declaration warning + unused var

fixed warnings:
```
  CC     cgminer-driver-opencl.o
driver-opencl.c: In function 'set_gpu_engine':
driver-opencl.c:318: warning: implicit declaration of function 'get_intrangeexitval'
driver-opencl.c: In function 'set_gpu_memclock':
driver-opencl.c:393: warning: implicit declaration of function 'get_intexitval'
driver-opencl.c:388: warning: unused variable 'valuesptr'
```