.gitignore: Ease /binaries pattern

The current /binaries/* hid the directory, but completely removed
it by "git-clean -d", which should not be intended.

Reported-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Michael Roth <mroth@nessie.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>