systemd/plasma-workspace: workaround for build race condition (MR 5327)

Explicitly generate qwayland-plasma-shell.h first, in order to fix:
  In file included from /usr/include/PlasmaQuick/PlasmaQuick/PlasmaShellWaylandIntegration:1,
                   from /home/pmos/build/src/plasma-workspace-6.1.2/shell/panelconfigview.cpp:35:
  /usr/include/PlasmaQuick/plasmaquick/plasmashellwaylandintegration.h:14:10: fatal error: qwayland-plasma-shell.h: No such file or directory
     14 | #include "qwayland-plasma-shell.h"
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.

Related: https://invent.kde.org/plasma/plasma-workspace/-/issues/132
[ci:skip-vercheck]
[ci:skip-build] times out after >3hr in gitlab CI

extra-repos/systemd/plasma-workspace/APKBUILD

@@ -127,6 +127,12 @@ build() {
 		-DCMAKE_INSTALL_LIBDIR=lib \
 		-DPLASMA_X11_DEFAULT_SESSION=OFF \
 		-DINSTALL_SDDM_WAYLAND_SESSION=OFF # Fails to get permissions to take control over tty
+
+	# workaround for:
+	# https://invent.kde.org/plasma/plasma-workspace/-/issues/132
+	ninja -C build shell/qwayland-plasma-shell.h
+	ninja -C build shell/wayland-plasma-shell-client-protocol.h
+
 	cmake --build build
 }