Add libdrm as a dependency of wlr_backend

Without this patch I'm getting the following build error when building
with Nix:

FAILED: backend/wlr_backend@sta/session_direct-ipc.c.o
In file included from ../backend/session/direct-ipc.c:20:0:
/nix/store/9ac27wk5vh47p28gladbdfafpidrx9rh-libdrm-2.4.88-dev/include/xf86drm.h:40:17: fatal error: drm.h: No such file or directory
 #include <drm.h>
                 ^
compilation terminated.

backend/meson.build

@@ -45,5 +45,5 @@ lib_wlr_backend = static_library(
 	'wlr_backend',
 	backend_files,
 	include_directories: wlr_inc,
-	dependencies: [wayland_server, egl, gbm, libinput, systemd, elogind, wlr_render, wlr_protos],
+	dependencies: [wayland_server, egl, gbm, libinput, systemd, elogind, wlr_render, wlr_protos, drm],
 )