Missing header for headless builds (quickfix, may not make much
sense to probe renderer on headless).

os_linuxbsd.cpp:249:46: error: 'RenderingServer' has not been declared

Caused by: https://github.com/godotengine/godot/commit/de768afbdcf
--- a/platform/linuxbsd/os_linuxbsd.cpp
+++ b/platform/linuxbsd/os_linuxbsd.cpp
@@ -34,4 +34,5 @@
 #include "main/main.h"
 #include "servers/display_server.h"
+#include "servers/rendering_server.h"
 
 #include "modules/modules_enabled.gen.h" // For regex.