Silence GCC 8 cast-function-type warnings

On Windows, casting between unrelated function types is fine and
sometimes even necessary, so just use an intermediate cast to
(void (*) (void)) to silence the warning as described in [0].

[0] https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html

Closes https://github.com/curl/curl/pull/2860