Add #ifdef gate around CL_KERNEL_ARG_TYPE_CONST (reported on missing on some...

Add #ifdef gate around CL_KERNEL_ARG_TYPE_CONST (reported on missing on some Windows ICD by Christoph Gohlke)

src/c_wrapper/wrap_constants.cpp

@@ -613,7 +613,10 @@ void populate_constants(void(*add)(const char*, const char*, int64_t value))
     // kernel_arg_type_qualifier
 #if PYOPENCL_CL_VERSION >= 0x1020
     ADD_ATTR("kernel_arg_type_qualifier", KERNEL_ARG_TYPE_, NONE);
+#ifdef CL_KERNEL_ARG_TYPE_CONST
+    // Apparently, this symbol is missing in some Windows ICD loaders.
     ADD_ATTR("kernel_arg_type_qualifier", KERNEL_ARG_TYPE_, CONST);
+#endif
     ADD_ATTR("kernel_arg_type_qualifier", KERNEL_ARG_TYPE_, RESTRICT);
     ADD_ATTR("kernel_arg_type_qualifier", KERNEL_ARG_TYPE_, VOLATILE);
 #endif