Fix compile warning in fix_freeze_kokkos

src/KOKKOS/fix_freeze_kokkos.h

@@ -49,11 +49,10 @@ class FixFreezeKokkos : public FixFreeze {
     }
 
     KOKKOS_INLINE_FUNCTION
-    volatile OriginalForce &operator+=(const volatile OriginalForce &rhs) volatile {
+    void operator+=(const volatile OriginalForce &rhs) volatile {
       values[0] += rhs.values[0];
       values[1] += rhs.values[1];
       values[2] += rhs.values[2];
-      return *this;
     }
   };