Ensure VBoxLinuxAdditions does not halt build

> virtualbox: Starting the VirtualBox Guest Additions ...done.
> virtualbox: Installing the Window System drivers ...fail!
> virtualbox: (Could not find the X.Org or XFree86 Window System.)

Though the above error in the VirtualBox Guest Additions Installer
may be 'safely' ignored, it results in the script returning a
non-zero exit status. This patch forces exit status to be ignored.

Signed-off-by: Alex Coles <alex@alexbcoles.com>