small fix

Author: ChrisZYJ

src/common/m_variables_conversion.fpp

@@ -1583,7 +1583,7 @@ contains
     end subroutine s_finalize_variables_conversion_module
 
 #ifndef MFC_PRE_PROCESS
-    subroutine s_compute_speed_of_sound(pres, rho, gamma, pi_inf, H, adv, vel_sum, c_c, c)
+    pure subroutine s_compute_speed_of_sound(pres, rho, gamma, pi_inf, H, adv, vel_sum, c_c, c)
 #ifdef _CRAYFTN
         !DIR$ INLINEALWAYS s_compute_speed_of_sound
 #else