tests: introduce a small hack for visibility

One of our testcases builds a small giomodule for testing the loading of
modules containing resources.  Unfortunately, this module gets built
using the same CFLAGS as the rest of GLib, including the visibility
flags (defaulting to hidden).

Use "config.h" to get a declaration of _GLIB_EXTERN that will export
symbols properly and use it to annotate the necessary APIs.

gio/tests/resourceplugin.c

@@ -1,16 +1,18 @@
+#include "config.h"
+
 #include <gio/gio.h>
 
-void
+_GLIB_EXTERN void
 g_io_module_load (GIOModule *module)
 {
 }
 
-void
+_GLIB_EXTERN void
 g_io_module_unload (GIOModule   *module)
 {
 }
 
-char **
+_GLIB_EXTERN char **
 g_io_module_query (void)
 {
   return NULL;