x11-toolkits/wlroots020: add missing dependency

$ pkg-config --libs wlroots-0.20
Package wayland-protocols was not found in the pkg-config search path.
Perhaps you should add the directory containing `wayland-protocols.pc'
to the PKG_CONFIG_PATH environment variable
Package 'wayland-protocols', required by 'wlroots-0.20', not found

https://gitlab.freedesktop.org/wlroots/wlroots/-/commit/122310a2de35
(cherry picked from commit ab987a3cdcc50d8e11ca49cf82dc0f5271fe53eb)