x11-wm/hyprland: unbreak plugin API after 54f2b0ad7dc5

In file included from hyprbars/barDeco.cpp:1:
In file included from hyprbars/barDeco.hpp:5:
In file included from /usr/local/include/hyprland/src/render/decorations/IHyprWindowDecoration.hpp:3:
In file included from /usr/local/include/hyprland/src/render/decorations/../../defines.hpp:1:
/usr/local/include/hyprland/src/render/decorations/../../includes.hpp:36:10: fatal error: 'wlr/backend.h' file not found
#include <wlr/backend.h>

^~~~~~~~~~~~~~~