Fixed a warning reported by CMake

The `cmake_minimum_required()` command should be called at the beginning
of the top-level `CMakeLists.txt` file even before calling the
`project()` command.

CMakeLists.txt

@@ -1,5 +1,5 @@
-project (KratosMultiphysics)
 cmake_minimum_required (VERSION 2.8.6)
+project (KratosMultiphysics)
 set (CMAKE_CXX_STANDARD 17)
 set (CMAKE_CXX_STANDARD_REQUIRED ON)
 set (CMAKE_CXX_EXTENSIONS OFF)