The libadwaita pkg-config adds `-I/usr/include/libadwaita-1` (or similar)
to the compiler flags, which means that
#include <adwaita.h>
is sufficient, and the currently used
#include <libadwaita-1/adwaita.h>
is not necessary.
Not only is it not necessary, but it prevents nautilus from being
built with an uninstalled libadwaita build (either meson subproject or
otherwise). To fix that, remove the `libadwaita-1/` prefix.
The libadwaita test programs also only `#include <adwaita.h>`, and
according to Debian Code Search, that is the more popular version.
https://codesearch.debian.net/search?q=%23include+%3Cadwaita.h%3E&literal=1
https://codesearch.debian.net/search?q=%23include+%3Clibadwaita-1%2Fadwaita.h%3E&literal=1
