build: fix possible race for some backends

There is missing dependency on linux-dmabuf-unstable-v1-server-protocol.h header file in backend-headless, backend-drm and backend-x11. That files do not depend on that header, in fact. But by this moment they've had that implicit dependency due to linux-dmabuf.h header.

With specific set of meson configure options the protocol header is not generated at the right time, what causes build error in 100% cases using small amount of building threads (from -j1 to -j8).

Meson configuration is here

-Dauto_features=enabled -Dbackend-default=drm -Dbackend-drm-screencast-vaapi=false -Dbackend-rdp=false -Dcolor-management-colord=false -Dcolor-management-lcms=false -Ddemo-clients=false -Dimage-jpeg=false -Dimage-webp=false -Dlauncher-logind=false -Dpipewire=false -Dremoting=false -Dsimple-clients= -Dsystemd=false -Dtest-junit-xml=false -Dwrap_mode=nodownload -Dxwayland=false

Can be reproduced with ninja -j 1 in 100% cases, reproduces "sometimes" with ninja -j 32.

Signed-off-by: Ivan Nikolaenko ivan.nikolaenko@unikie.com