Fix cmake for non top level inclusion

[sin3point14]

Description

I added ipctk as a submodule in my project and was getting the error:

CMake Error at vendor/ipc-toolkit/CMakeLists.txt:76 (set):
  set given invalid arguments: FORCE specified without CACHE


CMake Error at vendor/ipc-toolkit/CMakeLists.txt:77 (set):
  set given invalid arguments: FORCE specified without CACHE

These changes fix it

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added to my project's cmake using add_subdirectory and now cmake doesn't error.

Test Configuration:

[sin3point14@lolikawa ipc-toolkit]$ cat /etc/os-release 
NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling
ANSI_COLOR="38;2;23;147;209"
HOME_URL="https://archlinux.org/"
DOCUMENTATION_URL="https://wiki.archlinux.org/"
SUPPORT_URL="https://bbs.archlinux.org/"
BUG_REPORT_URL="https://gitlab.archlinux.org/groups/archlinux/-/issues"
PRIVACY_POLICY_URL="https://terms.archlinux.org/docs/privacy-policy/"
LOGO=archlinux-logo
[sin3point14@lolikawa ipc-toolkit]$ g++ --version
g++ (GCC) 15.1.1 20250425
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Checklist

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules