suppress unused typedef warning

Summary:
We're seeing the following compiler warnings:
```
unused type alias 'char_type' [-Werror,-Wunused-local-typedef]
```
and this breaks compilation since we build with `-Werror`.

I haven't fully understood why & when this warning is triggered (it doesn't always trigger), but by adding a `[[maybe_unused]]` we can suppress the warning.

Reviewed By: vitaut

Differential Revision: D38387281

fbshipit-source-id: a12c9791fcf3eb6bae72a78310ed461f882a9eb7

deps/fmt/include/fmt/format.h

@@ -2360,18 +2360,18 @@ FMT_CONSTEXPR void handle_dynamic_spec(int& value,
  }
 }
 
-#define FMT_STRING_IMPL(s, base, explicit) \
- [] { \
- /* Use the hidden visibility as a workaround for a GCC bug (#1973). */ \
- /* Use a macro-like name to avoid shadowing warnings. */ \
- struct FMT_GCC_VISIBILITY_HIDDEN FMT_COMPILE_STRING : base { \
- using char_type = fmt::remove_cvref_t<decltype(s[0])>;  \
- FMT_MAYBE_UNUSED FMT_CONSTEXPR explicit \
- operator fmt::basic_string_view<char_type>() const { \
- return fmt::detail_exported::compile_string_to_view<char_type>(s); \
- } \
- }; \
- return FMT_COMPILE_STRING(); \
+#define FMT_STRING_IMPL(s, base, explicit)  \
+ [] {  \
+ /* Use the hidden visibility as a workaround for a GCC bug (#1973). */  \
+ /* Use a macro-like name to avoid shadowing warnings. */  \
+ struct FMT_GCC_VISIBILITY_HIDDEN FMT_COMPILE_STRING : base {  \
+ using char_type FMT_MAYBE_UNUSED = fmt::remove_cvref_t<decltype(s[0])>; \
+ FMT_MAYBE_UNUSED FMT_CONSTEXPR explicit  \
+ operator fmt::basic_string_view<char_type>() const {  \
+ return fmt::detail_exported::compile_string_to_view<char_type>(s);  \
+ }  \
+ };  \
+ return FMT_COMPILE_STRING();  \
  }()
 
 /**