Raise cmake_minimum_required for ECM's CMake < 3.16 compatibility warning.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.16)
 project(fcitx5-libthai VERSION 5.1.10)
 
 set(REQUIRED_FCITX_VERSION 5.1.12)