mu-sexp: fix c++23 warning for suffix operator

Space between operator"" and _xxx is deprecated.

lib/utils/mu-sexp.hh

@@ -301,7 +301,7 @@ MU_ENABLE_BITOPS(Sexp::Format);
  * String-literal; allow for ":foo"_sym to be a symbol
  */
 static inline Sexp::Symbol
-operator"" _sym(const char* str, std::size_t n)
+operator""_sym(const char* str, std::size_t n)
 {
 	return Sexp::Symbol{str};
 }

meson.build

@@ -62,7 +62,10 @@ extra_flags = [
   '-Wno-switch-enum',
   # assuming these are false alarm... (in fmt, with gcc13):
   '-Wno-array-bounds',
-  '-Wno-stringop-overflow',]
+  '-Wno-stringop-overflow',
+  # c++23, in tabulate.hpp (3rd-party)
+  '-Wno-deprecated-literal-operator'
+]
 
 if (cxx.get_id() == 'clang')
   extra_flags += [