Make bionic's <netinet/icmp6.h> standalone, like glibc's.

This removes the need for a bionic-specific hack in external/iproute2.

Bug: 9671560
Change-Id: I9f15be0711d26bca863bd66be9a39606500fb948
diff --git a/libc/include/netinet/icmp6.h b/libc/include/netinet/icmp6.h
index 5a0293d..0ecc2e5 100644
--- a/libc/include/netinet/icmp6.h
+++ b/libc/include/netinet/icmp6.h
@@ -65,6 +65,8 @@
 #ifndef _NETINET_ICMP6_H_
 #define _NETINET_ICMP6_H_
 
+#include <netinet/in.h> /* android-added: glibc source compatibility. */
+
 #define ICMPV6_PLD_MAXLEN	1232	/* IPV6_MMTU - sizeof(struct ip6_hdr)
 					   - sizeof(struct icmp6_hdr) */