declare ret_val global variable static (KhronosGroup#147)

It would be better to remove all global ret_val variables, but
this would be a much bigger change.

Author: bashbaug

test/loader_test/test_kernel.c

@@ -15,7 +15,7 @@ extern cl_event event;
 extern cl_context  context;
 extern cl_command_queue command_queue;
 extern cl_device_id devices;
-int ret_val;
+static int ret_val;
 extern void CL_CALLBACK setevent_callback(cl_event _a, cl_int _b, void* _c);
 extern void CL_CALLBACK setprintf_callback(cl_context _a, cl_uint _b, char* _c, void* _d );