opusenc: fix segmentation fault at x86 version

The argument 0x0 is interpreted by the x86 compiler as a 32-bit int, but
it is consumed as a 64-bit uint causing a segmentation fault. We need to
explicit cast it to guint64 in order for the va_list to be built correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=797092

ext/opus/gstopusenc.c

diff --git a/ext/opus/gstopusenc.c b/ext/opus/gstopusenc.c
index 2282873..e1aa930 100644
--- a/ext/opus/gstopusenc.c
+++ b/ext/opus/gstopusenc.c
@@ -877,12 +877,12 @@
        * treated as a set of individual mono channels */
       s = gst_structure_copy (s2);
       gst_structure_set (s, "channels", G_TYPE_INT, i, "channel-mask",
-          GST_TYPE_BITMASK, 0x0, NULL);
+          GST_TYPE_BITMASK, G_GUINT64_CONSTANT(0), NULL);
       gst_caps_append_structure (caps, s);
 
       s = gst_structure_copy (s1);
       gst_structure_set (s, "channels", G_TYPE_INT, i, "channel-mask",
-          GST_TYPE_BITMASK, 0x0, NULL);
+          GST_TYPE_BITMASK, G_GUINT64_CONSTANT(0), NULL);
       gst_caps_append_structure (caps, s);
     }