Remove another volatile return type to avoid warning

src/KOKKOS/fix_langevin_kokkos.h

@@ -45,11 +45,10 @@ namespace LAMMPS_NS {
     }
 
     KOKKOS_INLINE_FUNCTION
-    volatile s_FSUM& operator+=(const volatile s_FSUM &rhs) volatile {
+    void operator+=(const volatile s_FSUM &rhs) volatile {
       fx += rhs.fx;
       fy += rhs.fy;
       fz += rhs.fz;
-      return *this;
     }
   };
   typedef s_FSUM FSUM;