Make FatFunction contents into proper ctype

Without this, it's possible for the macro-expanded code to trigger
improper_ctypes lints.

    error: `extern` block uses type `()`, which is not FFI-safe
        --> tests/ffi/lib.rs:2577:44
         |
    2577 |             fn __c_take_callback(callback: ::cxx::private::FatFunction);
         |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
         |
    note: the lint level is defined here
        --> tests/ffi/lib.rs:1439:8
         |
    1439 | #[deny(improper_ctypes, improper_ctypes_definitions)]
         |        ^^^^^^^^^^^^^^^
         = help: consider using a struct instead
         = note: tuples have unspecified layout
diff --git a/src/function.rs b/src/function.rs
index 1166b3d..647be6a 100644
--- a/src/function.rs
+++ b/src/function.rs
@@ -1,5 +1,7 @@
+use core::ffi::c_void;
+
 #[repr(C)]
 pub struct FatFunction {
-    pub trampoline: *const (),
-    pub ptr: *const (),
+    pub trampoline: *const c_void,
+    pub ptr: *const c_void,
 }