[build] Fix misaligned table entries when using gcc 4.5

Declarations without the accompanying __table_entry cause misalignment
of the table entries when using gcc 4.5.  Fix by adding the
appropriate __table_entry macro or (where possible) by removing
unnecessary forward declarations.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv6.c

@@ -15,8 +15,6 @@
 #include <ipxe/netdevice.h>
 #include <ipxe/if_ether.h>
 
-struct net_protocol ipv6_protocol;
-
 /* Unspecified IP6 address */
 static struct in6_addr ip6_none = {
         .in6_u.u6_addr32 = { 0,0,0,0 }