fix: silence CMake 4.x FetchContent_Populate deprecation warnings

Set CMP0169=OLD for deps that lack CMakeLists.txt (easywsclient,
tiny-process-library) and genuinely need FetchContent_Populate.

Author: 0xeb

CMakeLists.txt

@@ -1,4 +1,8 @@
 cmake_minimum_required(VERSION 3.16)
+# CMP0169: Allow FetchContent_Populate() for deps without CMakeLists.txt
+if(POLICY CMP0169)
+  cmake_policy(SET CMP0169 OLD)
+endif()
 project(fastmcpp VERSION 2.14.1 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 17)