libvbr/vbr.c: include <stdio.h> for vsnprintf()

This fixes an implicit declaration error with newer compilers and on
musl where stdio.h does not incidentally get included by some other
header.

Bug: https://bugs.gentoo.org/936591

Author: orlitzky

libvbr/vbr.c

@@ -18,6 +18,7 @@
 #include <arpa/inet.h>
 #include <arpa/nameser.h>
 #include <netdb.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>