Provide complete C prototypes for C-exported functions
A C function declaration with an empty parameter list does not declare
a function taking no parameters, but simply does not declares what
parameters it takes (hence, it's not a complete prototype). To denote
an empty parameter list, C uses the special `void` value.
This avoids warnings when building C sources including Scintilla.h if
using e.g. GCC's `-Wstrict-prototypes` option, as well as actually
disallowing passing parameters to those functions.