patch 8.1.2102: can't build with GTK and FEAT_GUI_GNOME

Problem:    Can't build with GTK and FEAT_GUI_GNOME. (Tony Mechelynck)
Solution:   Adjust the #ifdef. (Yegappan Lakshmanan)
diff --git a/src/version.c b/src/version.c
index 8feb2cc..f5b9d60 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2102,
+/**/
     2101,
 /**/
     2100,