Fix undeclared identifier error in lightmapper_cpu.cpp
Fix undeclared identifier error in lightmapper_cpu.cpp

Compiling with latest clang on macOS produced this error: use of undeclared identifier 'rand'.

Fixes godotengine/godot#45342

This file is not present in 4.0/master.

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot