Fix fallthrough attribute for older kernels.

src/ebt_igmpsnooping.c

@@ -29,6 +29,14 @@
 #include <linux/netfilter_bridge/ebt_igmpsnooping.h>
 #include <net/netfilter/br_netfilter.h>
 
+#ifndef fallthrough
+#if __has_attribute(__fallthrough__)
+# define fallthrough	__attribute__((__fallthrough__))
+#else
+# define fallthrough	do {} while (0)  /* fallthrough */
+#endif
+#endif
+
 static DEFINE_SPINLOCK(igmp_lock);
 
 static int igmp_snooping_show(struct seq_file *seq, void *v)