ITS GPU: INVALID_TRIGGER_ERROR_NO_HOST_CODE should not be defined away but GPUg() should only be used in GPUCode

[davidrohr]

@mconcas : GPUg() is a gpu keyword, not to be used in host code. Defining it away actually creates ambigious symbols: The host code will assume it is a normal CPU function, while it is in fact compiled as a GPU kernel.

I agree that there might be a use case to have the same function available as GPU-kernel and per thread on the CPU. But I think in that case, it is better to just wrap it in a kernel. Overall, I think we should not define INVALID_TRIGGER_ERROR_NO_HOST_CODE away, but it is a genuine error, and we should hide the kernel definitions in host code which does not understand it.

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