lkl: pull in linux/netlink.h into lkl headers

Add ipv6 neighbor entry needs netlink that depends on quite a lot on
linux/netlink.h. Code is messy if we define all of them by ourselves.

Signed-off-by: Yuan Liu <liuyuan@google.com>

arch/lkl/include/uapi/asm/syscalls.h

@@ -90,6 +90,7 @@ typedef __s64			s64;
 #include <asm/siginfo.h>
 #include <linux/utime.h>
 #include <asm/socket.h>
+#include <linux/netlink.h>
 
 /* Define data structures used in system calls that are not defined in UAPI
  * headers */