Add missing header for headless builds

platform/linuxbsd/os_linuxbsd.cpp

@@ -32,6 +32,7 @@
 
 #include "core/io/dir_access.h"
 #include "main/main.h"
+#include "servers/display_server.h"
 
 #ifdef X11_ENABLED
 #include "display_server_x11.h"