ls: add "pure" attribute

src/ls.hxx

@@ -20,6 +20,8 @@
 #ifndef MPD_LS_HXX
 #define MPD_LS_HXX
 
+#include "Compiler.h"
+
 #include <stdio.h>
 
 class Client;
@@ -29,6 +31,7 @@ class Client;
  * It is not allowed to pass an URI without a scheme, check with
  * uri_has_scheme() first.
  */
+gcc_pure
 bool uri_supported_scheme(const char *url);
 
 /**