account for LDA_X in VDW-DF2

test/xc_functionals/refactor_test/testuite-xc.ref.log

@@ -161,6 +161,10 @@ qe_xc: 0.00 VDW-DF2 ( 0 413 0 2)
   {
     "XC_functional_name": "LDA_C_PW"
   },
+  {
+    "XC_functional_name": "LDA_X",
+    "XC_functional_weight": -1.0
+  },
   {
     "XC_functional_name": "x_qe_VDW_DF2"
   }