fix a bug cuda cannot be compiled

Author: Qianruipku

source/src_pw/electrons.h

@@ -30,7 +30,8 @@ class Electrons: private Threshold_Elec
 
     double delta_total_energy;
 
-    // void self_consistent(const int &istep); // It is replaced by ESolver_KS::Run
+    // It should be replaced by ESolver_KS::Run. GPU still use it temperarily.
+    void self_consistent(const int &istep);
 
     void non_self_consistent(const int &istep);