MDEV-34206 compile failure: fmt use incompatible with libfmt-10.2.[2]+

Upstream libfmt commit https://github.com/fmtlib/fmt/commit/d70729215fba1d54862e407b626abf86ddf409bf
now requires the format function to be const.

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

sql/item_strfunc.cc

@@ -1382,7 +1382,7 @@ bool Item_func_sformat::fix_length_and_dec(THD *thd)
 namespace fmt {
   template <> struct formatter<String>: formatter<string_view> {
     template <typename FormatContext>
-    auto format(String c, FormatContext& ctx) -> decltype(ctx.out()) {
+    auto format(String c, FormatContext& ctx) const -> decltype(ctx.out()) {
       string_view name = { c.ptr(), c.length() };
       return formatter<string_view>::format(name, ctx);
     };