Ensure bridging header function prototypes are valid

Compiling my swift project I am seeing:

```
Generated/mycrateFFI.h:75:45: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
```

for the `ffi_mycrate_uniffi_contract_version` and
`uniffi_mycrate_checksum_func_myfun` declarations.

This is because they take no arguments which is emitted as `()`, for C `(void)`
is required, C++ is happy with either.

Add a specific case to `swift::arg_list_ffi_decl` to handle this. The resulting diff is:

```
--- mycrateFFI.h	2023-04-06 10:38:25.645130502 +0100
+++ mycrateFFI.h	2023-04-06 11:01:12.415958817 +0100
@@ -73,9 +73,11 @@ RustBuffer ffi_mycrate_rustbuffer_reserv
 );

 uint32_t ffi_mycrate_uniffi_contract_version(
+    void

 );

 uint16_t uniffi_mycrate_checksum_func_myfun(
+    void

 );
```

uniffi_bindgen/src/bindings/swift/templates/macros.swift

@@ -82,10 +82,14 @@
 // Note unfiltered name but ffi_type_name filters.
 -#}
 {%- macro arg_list_ffi_decl(func) %}
- {%- for arg in func.arguments() %}
- {{- arg.type_().borrow()|ffi_type_name }} {{ arg.name() -}}{% if !loop.last || func.has_rust_call_status_arg() %}, {% endif %}
- {%- endfor %}
- {%- if func.has_rust_call_status_arg() %}RustCallStatus *_Nonnull out_status{% endif %}
+ {%- if func.arguments().len() > 0 %}
+ {%- for arg in func.arguments() %}
+ {{- arg.type_().borrow()|ffi_type_name }} {{ arg.name() -}}{% if !loop.last || func.has_rust_call_status_arg() %}, {% endif %}
+ {%- endfor %}
+ {%- if func.has_rust_call_status_arg() %}RustCallStatus *_Nonnull out_status{% endif %}
+ {%- else %}
+ {%- if func.has_rust_call_status_arg() %}RustCallStatus *_Nonnull out_status{%- else %}void{% endif %}
+ {% endif %}
 {%- endmacro -%}
 
 {%- macro async(func) %}