virtualbox: 5.2.4 -> 5.2.6

Upstream changes without issue IDs:

 * GUI: fixed occasional screen corruption when host screen resolution
        is changed
 * User interface: increase proposed disk size when creating new VMs for
                   Windows 7 and newer
 * User interface: various improvements for high resolution screens
 * VMM: Fixed problems using 256MB VRAM in raw-mode VMs
 * Audio: implemented support for audio playback and recording for macOS
          guests
 * Audio: further timing improvements for Windows 10 guests
 * Linux hosts: fixed problem accessing mini-toolbar under XFCE

The full changelog including issue IDs can be found at:

https://www.virtualbox.org/wiki/Changelog#v6

What was not mentioned in the changelog is that this release fixes
compiling the VirtualBox modules against kernel 4.15, which was added in
commit 61043ad4

.

Tested this by running all of the tests in nixos/tests/virtualbox.nix.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @flokli, @svanderburg