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/session.c b/src/session.c
index 41835db..418fcba 100644
--- a/src/session.c
+++ b/src/session.c
@@ -985,7 +985,7 @@
     }
 }
 
-# if defined(USE_GNOME_SESSION) \
+# if defined(FEAT_GUI_GNOME) \
 	|| (defined(GUI_MAY_SPAWN) && defined(EXPERIMENTAL_GUI_CMD)) \
 	|| defined(PROTO)
 /*