patch 8.0.1256: typo in configure variable vim_cv_tgent

Problem:    Typo in configure variable vim_cv_tgent. (Matthieu Guillard)
Solution:   Rename the variable. (closes #2281)
diff --git a/src/version.c b/src/version.c
index 8cc9bba..e94b81a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1256,
+/**/
     1255,
 /**/
     1254,