patch 8.0.0557: GTK: using static gravities is not useful

Problem:    GTK: using static gravities is not useful.
Solution:   Remove setting static gravities. (Kazunobu Kuriyama)
diff --git a/src/version.c b/src/version.c
index efb2ef3..d37033f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    557,
+/**/
     556,
 /**/
     555,