net: add missing include in include/net/gro.h

This is needed for some arches, as reported by Geert Uytterhoeven,
Randy Dunlap and Stephen Rothwell

Fixes: 4721031 ("net: move gro definitions to include/net/gro.h")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20211117100130.2368319-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/gro.h

@@ -6,6 +6,7 @@
 #include <linux/indirect_call_wrapper.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <net/ip6_checksum.h>
 #include <linux/skbuff.h>
 #include <net/udp.h>