patch 8.0.0620: checking for HAVE_GTK_MULTIHEAD is not needed

Problem:    Since we only support GTK versions that have it, the ckeck for
            HAVE_GTK_MULTIHEAD is no longer needed.
Solution:   Remove HAVE_GTK_MULTIHEAD. (Kazunobu Kuriyama)
diff --git a/src/version.c b/src/version.c
index 3d91495..6e232dc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    620,
+/**/
     619,
 /**/
     618,