Fluid: Fix strict compiler warning

Proper way to check for DEBUG_PRINT is to do `if defined` check rather
than `if`: this is because in non-debug builds the symbol is not defined.

source/blender/blenkernel/intern/fluid.c

@@ -3207,9 +3207,12 @@ static void update_effectors_task_cb(void *__restrict userdata,
       data->force_y[index] = retvel[1];
       data->force_z[index] = retvel[2];
 
-#  if DEBUG_PRINT
+#  ifdef DEBUG_PRINT
       /* Debugging: Print forces. */
-      printf("setting force: [%f, %f, %f]\n", data->force_x[index], data->force_y[index], data->force_z[index]);
+      printf("setting force: [%f, %f, %f]\n",
+             data->force_x[index],
+             data->force_y[index],
+             data->force_z[index]);
 #  endif
     }
   }