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 c473461d (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: c473461dSigned-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -172,8 +172,11 @@ lib: $(VERSION_HEADER) config built-in.o
 	$(Q) $(MAKE) $(build)=lib all
 
 ifeq ($(VDSO),y)
+$(ARCH_DIR)/vdso-pie.o: pie
+	$(Q) $(MAKE) $(build)=pie $(ARCH_DIR)/vdso-pie.o
 PROGRAM-BUILTINS	+= $(ARCH_DIR)/vdso-pie.o
 endif
+
 PROGRAM-BUILTINS	+= pie/util-fd.o
 PROGRAM-BUILTINS	+= pie/util.o
 PROGRAM-BUILTINS	+= protobuf/built-in.o