Fenstergröße VirtualBox Debian

stephan@debian:~$ su
Passwort: 
root@debian:/home/stephan# mount /dev/sr0 /mnt
mount: /dev/sr0 is write-protected, mounting read-only
mount: /dev/sr0 is already mounted or /mnt busy
       /dev/sr0 is already mounted on /mnt
root@debian:/home/stephan# cd /mnt
root@debian:/mnt# ./VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.2.18 Guest Additions for Linux........
VirtualBox Guest Additions installer
Removing installed version 5.2.18 of VirtualBox Guest Additions...
Copying additional installer modules ...
Installing additional modules ...
VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel modules.  This may take a while.
This system is currently not set up to build kernel modules.
Please install the Linux kernel "header" files matching the current kernel
for adding new hardware support to the system.
The distribution packages containing the headers are probably:
    linux-headers-amd64 linux-headers-4.9.0-7-amd64
VirtualBox Guest Additions: Starting.
VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel modules.  This may take a while.
This system is currently not set up to build kernel modules.
Please install the Linux kernel "header" files matching the current kernel
for adding new hardware support to the system.
The distribution packages containing the headers are probably:
    linux-headers-amd64 linux-headers-4.9.0-7-amd64
root@debian:/mnt#