[TCP]: Put back tcp_timer_bug_msg[] symbol export.

It is needed for tcp_reset_xmit_timer(), which is invoked by
tcp_prequeue() which is invoked from tcp_ipv6.c
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -38,6 +38,7 @@ static void tcp_keepalive_timer (unsigned long data);
 
 #ifdef TCP_DEBUG
 const char tcp_timer_bug_msg[] = KERN_DEBUG "tcpbug: unknown timer value\n";
+EXPORT_SYMBOL(tcp_timer_bug_msg);
 #endif
 
 /*