opus: Rename opusheader functions to prevent symbol conflicts with static linking

The opus plugin in -base has the same functions.

ext/opus/Makefile.am

@@ -6,7 +6,10 @@ libgstopusparse_la_CFLAGS = \
 	$(GST_PLUGINS_BAD_CFLAGS) \
 	$(GST_PLUGINS_BASE_CFLAGS) \
 	$(GST_CFLAGS) \
-	$(OPUS_CFLAGS)
+	$(OPUS_CFLAGS) \
+	-Dgst_opus_header_is_header=gst_opus_bad_header_is_header \
+	-Dgst_opus_header_is_id_header=gst_opus_bad_header_is_id_header \
+	-Dgst_opus_header_is_comment_header=gst_opus_bad_header_is_comment_header
 libgstopusparse_la_LIBADD = \
 	$(GST_PLUGINS_BASE_LIBS) -lgstaudio-$(GST_API_VERSION) \
 	-lgsttag-$(GST_API_VERSION) \