nvidia-x11: stable 387.34 -> 390.25
nvidia-x11: stable 387.34 -> 390.25

390.x is Nvidia's latest "Long Lived Branch version" according to
https://www.nvidia.com/object/unix.html so this upgrades the stable version
from 387.xx.

390.x also also has support for kernel 4.15 and later (due to removal of the
old init_timer APIs, among other things), meaning that
linuxPackages_4_15.nvidia_x11 now builds correctly.

Signed-off-by: Austin Seipp <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

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