Declare a minimum CMake version in the subdirectory.

The ebuild configures from src/, so this file is the top level as far as
CMake is concerned, and CMake 4 refuses to configure a project that does not
state a minimum version. The repository root already declares one for the
normal build.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,3 +1,5 @@
+cmake_minimum_required(VERSION 3.10)
+
 cmake_policy(SET CMP0091 NEW)
 set(CMAKE_POLICY_DEFAULT_CMP0091 NEW)
 set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")