Don't create prebuilts for files that don't exist

make checkbuild was ignoring prebuilts, so it didn't flag prebuilts with
missing source files.  Don't create prebuilts for files that don't exist
so checkbuild can start treating it as an error.

Bug: 32332829
Test: m -j checkbuild
Change-Id: I868f98c8a13aaca6d967c4e33f323c0f931fbeac
1 file changed