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.

modules/lightmapper_cpu/lightmapper_cpu.cpp

@@ -777,7 +777,7 @@ void LightmapperCPU::_compute_direct_light(uint32_t p_idx, void *r_lightmap) {
 
 _ALWAYS_INLINE_ float uniform_rand() {
 	/* Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs" */
-	static thread_local uint32_t state = rand();
+	static thread_local uint32_t state = Math::rand();
 	state ^= state << 13;
 	state ^= state >> 17;
 	state ^= state << 5;