patch 8.2.2304: Vim9: no test for unletting an imported variable

Problem:    Vim9: no test for unletting an imported variable.
Solution:   Add a test.  Fix line number in error.
diff --git a/src/version.c b/src/version.c
index a2553fc..77f032d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2304,
+/**/
     2303,
 /**/
     2302,