Declare gDeviceType in clcpp image test

Fixes build on macOS. This is a hangover from some Apple specific code
that is in the image helper functions.xs

test_conformance/clcpp/images/main.cpp

@@ -19,6 +19,10 @@
 #include "test_sample.hpp"
 #include "test_write.hpp"
 
+// FIXME: To use certain functions in test_common/harness/imageHelpers.h
+// (for example, generate_random_image_data()), the tests are required to declare
+// the following variable (hangover from code specific to Apple's implementation):
+cl_device_type gDeviceType = CL_DEVICE_TYPE_DEFAULT;
 
 int main(int argc, const char *argv[])
 {