patch 8.0.0616: not always setting 'background' correctly after :hi Normal

Problem:    When setting the cterm background with ":hi Normal" the value of
            'background' may be set wrongly.
Solution:   Check that the color is less than 16.  Don't set 'background' when
            it was set explicitly. (Lemonboy, closes #1710)
diff --git a/src/version.c b/src/version.c
index c0711fb..eb46948 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    616,
+/**/
     615,
 /**/
     614,