patch 8.0.0317: no test for setting 'guifont'

Problem:    No test for setting 'guifont'.
Solution:   Add a test for X11 GUIs. (Kazunobu Kuriyama)
diff --git a/src/version.c b/src/version.c
index 68af6ff..50c6fb1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    317,
+/**/
     316,
 /**/
     315,