libpayload: Add pci_free_dev() and some boilerplate

Add just enough code and boilerplate to keep it compatible with future
libflashrom.

Change-Id: If0d46fab141da525f8f115d3f6045a8c417569eb
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/20955
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/libpci/libpci.c

@@ -219,3 +219,8 @@ struct pci_dev *pci_get_dev(struct pci_access* pacc, u16 domain, u8 bus, u8 dev,
 	cur->func = func;
 	return cur;
 }
+
+void pci_free_dev(struct pci_dev *const dev)
+{
+	free(dev);
+}