equalizer: Fix -Wincompatible-pointer-types warning

This is caused by the new type propagation for g_object_ref.

https://bugzilla.gnome.org/show_bug.cgi?id=791494
diff --git a/gst/equalizer/gstiirequalizer.c b/gst/equalizer/gstiirequalizer.c
index eb23d21..210296b 100644
--- a/gst/equalizer/gstiirequalizer.c
+++ b/gst/equalizer/gstiirequalizer.c
@@ -323,7 +323,7 @@
     g_return_val_if_fail (index < equ->freq_band_count, NULL);
   }
 
-  ret = g_object_ref (equ->bands[index]);
+  ret = g_object_ref (G_OBJECT (equ->bands[index]));
   BANDS_UNLOCK (equ);
 
   GST_LOG_OBJECT (equ, "return child[%d] %" GST_PTR_FORMAT, index, ret);