Prepend "Opus" to the version string.

This lets users print it out without context, which is a little
easier for clients.

src/opus.c

@@ -55,5 +55,5 @@ const char *opus_strerror(int error)
 
 const char *opus_get_version_string(void)
 {
-	return OPUS_VERSION;
+	return "Opus " OPUS_VERSION;
 }