CMake: 'codegen' is a reserved target name

[nic11]

CMake Warning (dev) at skyrim-platform/src/platform_se/CMakeLists.txt:161 (add_custom_target):
  Policy CMP0171 is not set: 'codegen' is a reserved target name.  Run "cmake
  --help-policy CMP0171" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  The target name "codegen" is reserved.
This warning is for project developers.  Use -Wno-dev to suppress it.