lto: Fix missing cleanup with incremental LTO.

Incremental LTO disabled cleanup of output_files since they have to
persist in ltrans cache.
This unintetionally also kept temporary early debug "*.debug.temp.o"
files.

Bootstrapped/regtested on x86_64-linux.
Ok for trunk?

lto-plugin/ChangeLog:

	* lto-plugin.c (cleanup_handler): Keep only files in ltrans
	cache.