[CMake] Fails to install under non-default prefix

[jbeich]

Regressed by #154 due to using CMAKE_INSTALL_DATAROOTDIR without include(GNUInstallDirs).

$ cmake -DCMAKE_INSTALL_PREFIX:PATH=/tmp/xdg-desktop-portal-hyprland_prefix -B /tmp/xdg-desktop-portal-hyprland_build -G Ninja
$ cmake --build /tmp/xdg-desktop-portal-hyprland_build
$ cmake --install /tmp/xdg-desktop-portal-hyprland_build
-- Install configuration: ""
-- Installing: /tmp/xdg-desktop-portal-hyprland_prefix/bin/hyprland-share-picker
-- Set non-toolchain portion of runtime path of "/tmp/xdg-desktop-portal-hyprland_prefix/bin/hyprland-share-picker" to ""
-- Up-to-date: /tmp/xdg-desktop-portal-hyprland_prefix/bin/hyprland-share-picker
-- Installing: /tmp/xdg-desktop-portal-hyprland_prefix/bin/xdg-desktop-portal-hyprland
-- Set non-toolchain portion of runtime path of "/tmp/xdg-desktop-portal-hyprland_prefix/bin/xdg-desktop-portal-hyprland" to ""
CMake Error at /tmp/xdg-desktop-portal-hyprland_build/cmake_install.cmake:94 (file):
  file cannot create directory: /xdg-desktop-portal/portals.  Maybe need
  administrative privileges.

[fufexan]

Fixed in 86a94c3.

[jbeich]

I confirm, installs fine now.