ensure tar only includes one copy of each preload file

I got an error down in the guts of BrowserFS because there was a second copy of `java/lang/Runtime.class` in `mini-rt.tar` that was a symlink.  Since it was zero bytes long, the attempt to check for `0xCAFEBABE` at the beginning of it failed with a RangeError (in FF).  There may be some issue further up the chain that caused `Runtime.class` to get listed twice in `tools/preload`, but this works around it nicely.