Add missing call to 'virtual-target.register' when symlinking targets.

Otherwise, if we build 'install' target twice, we'll get two different
symlink targets.


[SVN r30400]

tools/build

@@ -1 +1 @@
-Subproject commit 7320372b2cd44d59e7663275bea0b88b5905040c
+Subproject commit 82ca771e1143d6c819ff8fe878b4f041ca8b1c94