Fix: z_endstop_adj is never initialized

Marlin/configuration_store.cpp

 
   #elif ENABLED(Z_DUAL_ENDSTOPS)
 
-    float z_endstop_adj =
+    z_endstop_adj =
       #ifdef Z_DUAL_ENDSTOPS_ADJUSTMENT
         Z_DUAL_ENDSTOPS_ADJUSTMENT
       #else