meson: Fix use of install_data() without specifying install_dir
When building as a subproject, Meson emits the following warning:
subprojects/wayland-1.23.0/meson.build:130: WARNING: Project targets '>= 0.57.0' but uses feature introduced in '1.3.0': install_data() without install_dir inside of a subproject. This was broken and would install to the project name of the parent project instead
This MR fixes that.