Make vtables non-zero-size to fix a rustc warning.

```
warning: found non-foreign-function-safe member in struct marked #[repr(C)]: found zero-size struct in foreign module, consider adding a member to this struct
```

Author: SimonSapin

src/codegen/mod.rs

@@ -716,7 +716,9 @@ impl<'a> CodeGenerator for Vtable<'a> {
             .item()
             .pub_()
             .with_attrs(attributes)
-            .struct_(self.canonical_name(ctx))
+            .tuple_struct(self.canonical_name(ctx))
+            .field()
+            .build_ty(helpers::ast_ty::raw_type(ctx, "c_void"))
             .build();
         result.push(vtable);
     }

tests/expectations/tests/enum_and_vtable_mangling.rs

@@ -11,8 +11,7 @@ pub const whatever_else: _bindgen_ty_1 = _bindgen_ty_1::whatever_else;
 pub enum _bindgen_ty_1 { match_ = 0, whatever_else = 1, }
 #[repr(C)]
 #[derive(Default)]
-pub struct C__bindgen_vtable {
-}
+pub struct C__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct C {

tests/expectations/tests/nested_vtable.rs

@@ -6,8 +6,7 @@
 
 #[repr(C)]
 #[derive(Default)]
-pub struct nsISupports__bindgen_vtable {
-}
+pub struct nsISupports__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct nsISupports {

tests/expectations/tests/ref_argument_array.rs

@@ -7,8 +7,7 @@
 pub const NSID_LENGTH: ::std::os::raw::c_uint = 10;
 #[repr(C)]
 #[derive(Default)]
-pub struct nsID__bindgen_vtable {
-}
+pub struct nsID__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct nsID {

tests/expectations/tests/virtual_dtor.rs

@@ -6,8 +6,7 @@
 
 #[repr(C)]
 #[derive(Default)]
-pub struct nsSlots__bindgen_vtable {
-}
+pub struct nsSlots__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug)]
 pub struct nsSlots {

tests/expectations/tests/virtual_inheritance.rs

@@ -26,8 +26,7 @@ impl Clone for A {
 }
 #[repr(C)]
 #[derive(Default)]
-pub struct B__bindgen_vtable {
-}
+pub struct B__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct B {
@@ -54,8 +53,7 @@ impl Default for B {
 }
 #[repr(C)]
 #[derive(Default)]
-pub struct C__bindgen_vtable {
-}
+pub struct C__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct C {

tests/expectations/tests/virtual_overloaded.rs

@@ -6,8 +6,7 @@
 
 #[repr(C)]
 #[derive(Default)]
-pub struct C__bindgen_vtable {
-}
+pub struct C__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct C {

tests/expectations/tests/vtable_recursive_sig.rs

@@ -24,8 +24,7 @@ impl Default for Derived {
 }
 #[repr(C)]
 #[derive(Default)]
-pub struct Base__bindgen_vtable {
-}
+pub struct Base__bindgen_vtable(::std::os::raw::c_void);
 #[repr(C)]
 #[derive(Debug, Copy)]
 pub struct Base {