[NET]: Need struct sock forward decl in net/compat.h

Else we get build failures like:

  CC      arch/sparc64/kernel/sparc64_ksyms.o
In file included from arch/sparc64/kernel/sparc64_ksyms.c:28:
include/net/compat.h:37: warning: "struct sock" declared inside parameter list
include/net/compat.h:37: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: David S. Miller <davem@davemloft.net>