Set c++ version to c++23

I followed @pennae's advice and moved the constructor definition of
`AttrName` from the header file `nixexpr.hh` to `nixexpr.cc`.

Change-Id: I733f56c25635b366b11ba332ccec38dd7444e793

src/libexpr/nixexpr.cc

@@ -21,6 +21,14 @@ std::ostream & operator <<(std::ostream & str, const SymbolStr & symbol)
     return printIdentifier(str, s);
 }
 
+AttrName::AttrName(Symbol s) : symbol(s)
+{
+}
+
+AttrName::AttrName(std::unique_ptr<Expr> e) : expr(std::move(e))
+{
+}
+
 void Expr::show(const SymbolTable & symbols, std::ostream & str) const
 {
     abort();

src/libexpr/nixexpr.hh

@@ -30,8 +30,8 @@ struct AttrName
 {
     Symbol symbol;
     std::unique_ptr<Expr> expr;
-    AttrName(Symbol s) : symbol(s) {};
-    AttrName(std::unique_ptr<Expr> e) : expr(std::move(e)) {};
+    AttrName(Symbol s);
+    AttrName(std::unique_ptr<Expr> e);
 };
 
 typedef std::vector<AttrName> AttrPath;