build: Consistently include libglnx header as "libglnx.h"

Recent Meson versions have warnings if you add the subprojects directory as an include path, because the way Meson wants to consume subprojects is by the subproject's build system producing a Meson dependency object that encapsulates its include directory.

libglnx seems to be set up to have the libglnx directory be its include path instead: for example, ostree (by the author of libglnx) already uses "libglnx.h" or <libglnx.h> everywhere. Do the same here.

Flatpak equivalent: https://github.com/flatpak/flatpak/pull/4842