patch 8.2.3067: building fails with Athena

Problem:    Building fails with Athena. (Elimar Riesebieter)
Solution:   Adjust #ifdefs and add the 'drop_file' feature.
diff --git a/src/testing.c b/src/testing.c
index b32eb1a..f01f73a 100644
--- a/src/testing.c
+++ b/src/testing.c
@@ -1260,7 +1260,7 @@
     void
 f_test_gui_drop_files(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
 {
-# ifdef FEAT_GUI
+#if defined(HAVE_DROP_FILE)
     int		row;
     int		col;
     int_u	mods;