[RFC] Use strict function prototypes

`-Wstrict-prototypes` warn if a function is declared or defined without
specifying the argument types.

This warning disallow function prototypes with empty parameter list.
In C, a function declared with an empty parameter list accepts an
arbitrary number of arguments when being called. This is for historic
reasons; originally, C functions didn't have prototypes, as C evolved
from B, a typeless language. When prototypes were added, the original
typeless declarations were left in the language for backwards
compatibility.
Instead we should provide `void` in argument list to state
that function doesn't have arguments.

Also this warning disallow declaring type of the parameters after the
parentheses because Neovim's header generator doesn't use this
information and produce typeless declarations.
Actually doxygen output doesn't contain documentation for this function (and all functions in `regexp_nfa.c`). So I can't check it. Do you have any suggestion how to format it better?