Bugfix: opencl: Declare opencl_dynamic_cleanup in header

driver-opencl.h

@@ -20,6 +20,7 @@ extern char *set_vector(char *arg);
 extern char *set_worksize(char *arg);
 extern char *set_kernel(char *arg);
 void manage_gpu(void);
+extern void opencl_dynamic_cleanup();
 extern void pause_dynamic_threads(int gpu);
 
 extern bool have_opencl;