build: Do not specify a directory as an output file

This has strange behaviors, since timestamps of directories don't work
like timestamps of files. Ninja also expects all output files to be
files, not directories, otherwise it can crash in some cases. I'll be
fixing ninja to complain about this more proactively.

Test: treehugger
Change-Id: I8fc293aeba6f76623fdd93225900124ce8082940
1 file changed