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.
diff --git a/src/version.c b/src/version.c
index c625b05..33bae20 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1668,
+/**/
     1667,
 /**/
     1666,