DragonFly/FreeBSD: Make ss_sp *mut c_void as in the other implementations

This fixes compiling Rust.

src/unix/bsd/freebsdlike/mod.rs

@@ -81,7 +81,7 @@ s! {
     }
 
     pub struct stack_t {
-        pub ss_sp: *mut ::c_char,
+        pub ss_sp: *mut ::c_void,
         pub ss_size: ::size_t,
         pub ss_flags: ::c_int,
     }