[libxml2] Update minimum required CMake version

[Nick Wellnhofer <nwellnhof src gnome org>]

commit f93ca3e140a371b26366f747a408588c631e0fd1
Author: Markus Rickert <rickert fortiss org>
Date:   Fri Jan 15 17:53:27 2021 +0100

    Update minimum required CMake version

 CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6b67010a..da0e5a91 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14)
+cmake_minimum_required(VERSION 3.15)
 
 project(libxml2 C)