This adds a few previously missing generated files to .gitignore: the
qemu-bridge-helper binary, and more generated versions of the
linuxboot, multiboot and kvmvapic roms from pc-bios/optionrom.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>