lib/fmt: support build with libfmt-11.0.0

Upstream libfmt commit fmtlib/fmt@d707292
now requires the format function to be const.

Adjust the function prototype so it is const and can compile.

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>

src/lib/ffmpeg/LibFmt.hxx

@@ -13,7 +13,7 @@ template<>
 struct fmt::formatter<AVSampleFormat> : formatter<string_view>
 {
 	template<typename FormatContext>
-	auto format(const AVSampleFormat format, FormatContext &ctx) {
+	auto format(const AVSampleFormat format, FormatContext &ctx) const {
 		const char *name = av_get_sample_fmt_name(format);
 		if (name == nullptr)
 			name = "?";