make: fix "make criu" after arch-specific vdso broke it

Building criu with "make criu" on a clean tree was not working, failing on:

  make[1]: *** No rule to make target `arch/x86/vdso-pie.o'.  Stop.
  make: *** [arch/x86/vdso-pie.o] Error 2

git bisect traced the regression to commit c473461d24fd (vdso: Make it arch
specific) which apparently dropped the rule to build $(ARCH_DIR)/vdso-pie.o
using the pie rule.  Restore the dependency for "make criu" to work again from
a clean tree.

Tested:
$ git clean -fdx
$ make criu

Fixes: c473461d24fdfcd25542b427829a37fd2f0facb5

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>