lib/fmt: support build with libfmt-11.0.0
1402869715e3efca87942d79c3173a6b21a6925d

Index: src/lib/fmt/PathFormatter.hxx
--- src/lib/fmt/PathFormatter.hxx.orig
+++ src/lib/fmt/PathFormatter.hxx
@@ -29,7 +29,7 @@ template<>
 struct fmt::formatter<Path> : formatter<string_view>
 {
 	template<typename FormatContext>
-	auto format(Path path, FormatContext &ctx) {
+	auto format(Path path, FormatContext &ctx) const {
 		return formatter<string_view>::format(path.ToUTF8(), ctx);
 	}
 };
