openjdk: fix build for !enableGnome2

This OpenJDK packaging has a headless build configuration controlled by
the `minimal` flag, which is regularly build-tested by Hydra, and a
non-headless configuration based on pure Xlib libraries without Gnome
features, which is not normally tested.

Sometime before OpenJDK 8, the !enableGnome2 case broke, because it
needs to link against libXrandr but that wasn't included in the
buildInputs.

If this patch is backported to NixOS 18.03 or earlier, the same fix
needs to be applied to OpenJDK 9.

I have tested OpenJDK versions 8, 9, and 10, but not any other versions.