package/libassuan: fixup libassuan-config

The libassuan library installs libassuan-config, but it is not
currently fixed up by our <pkg>_CONFIG_SCRIPTS logic. While this
doesn't cause any problem today, it fails badly with per-package
folders, so let's use <pkg>_CONFIG_SCRIPTS.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>