Makefile.install: fix uninstall target

It was not working:

> $ make DESTDIR=`pwd`/inst3 uninstall
>  UNINSTALL crit.1
>  UNINSTALL criu.8
>  UNINSTALL libcriu.so
>  UNINSTALL pkgconfig/criu.pc
>  UNINSTALL crit
> /media/ts/kir/git/criu/scripts/nmk/scripts/build.mk:83: criu/arch/x86/Makefile: Not a directory
> make[2]: *** No rule to make target 'criu/arch/x86/Makefile'.  Stop.
> Makefile:48: recipe for target 'criu/arch/x86/crtools.built-in.o' failed
> make[1]: *** [criu/arch/x86/crtools.built-in.o] Error 2
> Makefile.install:49: recipe for target 'uninstall' failed
> make: *** [uninstall] Error 2

This is a fix to commit 7a360484 ("build/make: return to make from top
directory").
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile.install

@@ -43,5 +43,5 @@ install: install-man install-lib install-criu
 uninstall:
 	$(Q) $(MAKE) -C Documentation $@
 	$(Q) $(MAKE) $(build)=lib $@
-	$(Q) $(MAKE) -C criu $@
+	$(Q) $(MAKE) $(build)=criu $@
 .PHONY: uninstall