[CN] feature: allow mention of function pointers with empty parameter lists

[peterohanley]

struct foo {
    int (*bar)();
};

% cn verify empty_param.c 
empty_param.c:1:1: error: C-type signed int ()*
struct foo {
~~~~~~~^~~~~ 

This is holding up TA2 work. We don't need to be able to call these functions at this time. I expect calling them will require an ownership that includes an actual type for the pointer.

[yav]

I'll have a go at fixing this. In the meantime, adding an explicit void to the parameter list seems to make things work, in case this is an option.

[cp526]

@kmemarian comments: as noted on the PR ( #459 ) this isn't completely fixed yet: some change of Cerberus's handling of FunctionNoParams is needed in the frontend.