patch 9.0.1993: warning about unused function definition

Problem:  warning about unused function definition
Solution: add ifdefs

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optionstr.c b/src/optionstr.c
index 6b40dcd..202e93e 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -849,6 +849,7 @@
     return ret;
 }
 
+# if defined(FEAT_GUI_MSWIN) || defined(FEAT_GUI_GTK)
 static garray_T *expand_cb_ga;
 static optexpand_T *expand_cb_args;
 
@@ -938,6 +939,7 @@
     *numMatches = ga.ga_len;
     return OK;
 }
+#endif
 
 /*
  * Expand an option which is a list of flags.