patch 8.0.1668: terminal debugger: can't re-open source code window

Problem:    Terminal debugger: can't re-open source code window.
Solution:   Add the :Source command.  Also create the window if needed when
            gdb stops at a source line.

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1668,
 /**/
     1667,
 /**/