virtual_device.fragment: Disable signing of modules

Enabling module signing support for gki kernel configs
causing merge configs to sign the modules as well but
do not have temporary keys to do so; causing build
failures.

Failure was seen with kleaf target:
//common-modules/virtual-device:virtual_device_x86_64_dist

Bug: 200082547
Test: TH, manual kleaf build passed which was failing
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: Ic83fc1e12c087c74f18cc3eb1378f1f04c50ef80