cuda.lang: test the kernel-call context

When parsing the cuda.lang file there is a warning about the
style-ref="latex:display-math", but it actually works (with a style
scheme that defines a style for latex:display-math, such as tango and
classic).

The code is taken from (it is not the complete function):
http://docs.nvidia.com/cuda/cuda-samples/#cuda-parallel-reduction

https://bugzilla.gnome.org/show_bug.cgi?id=739684