patch 8.0.1320: popup test fails on GUI-only build

Problem:    Popup test fails on GUI-only build.
Solution:   Don't test balloon_split() when it's not available.
diff --git a/src/version.c b/src/version.c
index 2e52e80..324a360 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1320,
+/**/
     1319,
 /**/
     1318,