Fix various const-correctness issues

Spotted when temporarily compiling with -Wwrite-strings. This only goes
a small way towards making the code base -Wwrite-strings–clean. It
introduces no functional changes, and fixes no bugs.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: nobody

gio/tests/file.c

@@ -153,9 +153,9 @@ typedef struct
   gint monitor_created;
   gint monitor_deleted;
   gint monitor_changed;
-  gchar *monitor_path;
+  const gchar *monitor_path;
   gint pos;
-  gchar *data;
+  const gchar *data;
   gchar *buffer;
   guint timeout;
 } CreateDeleteData;