Fix build error without unity build after recent changes

float3 should have been declared within the blender namespace. And forward
declaration is difficult with templated classes so just include header.

source/blender/geometry/GEO_mesh_primitive_cuboid.hh

@@ -2,8 +2,9 @@
 
 #pragma once
 
+#include "BLI_math_vec_types.hh"
+
 struct Mesh;
-struct float3;
 namespace blender {
 namespace bke {
 class AttributeIDRef;