um: syscall.c: include <asm/unistd.h>

Without CONFIG_SECCOMP, we don't get this include recursively
through the existing includes, thus failing the build on not
having __NR_syscall_max defined. Add the necessary include to
fix this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>