Fixed some OpenACC directives (MFlowCode#859)

Author: prathi-wind

src/common/m_finite_differences.fpp

@@ -18,8 +18,7 @@ contains
         integer :: x, y, z !< Generic loop iterators
 
         real(wp) :: divergence
-
-        $:PARALLEL_LOOP(collapse=3, private=["divergence"])
+        $:PARALLEL_LOOP(collapse=3, gang vector default(present) private=["divergence"])
         do x = ix_s%beg, ix_s%end
             do y = iy_s%beg, iy_s%end
                 do z = iz_s%beg, iz_s%end