[all] use explicit void parameter lists

This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.

Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.

client/fonts/SDL/src/sdl.c

@@ -150,4 +150,4 @@ struct LG_Font LGF_SDL =
   .destroy      = lgf_sdl_destroy,
   .render       = lgf_sdl_render,
   .release      = lgf_sdl_release
-};
+};