Build: fix CMake warning about calling cmake_minimum_required

Fix the following CMake warning:
```
CMake Warning (dev) at CMakeLists.txt:1 (PROJECT):
  cmake_minimum_required() should be called prior to this top-level project()
  call.  Please see the cmake-commands(7) manual for usage documentation of
  both commands.
This warning is for project developers.  Use -Wno-dev to suppress it.
```

CMakeLists.txt

@@ -1,7 +1,7 @@
-PROJECT(tntcxx)
-
 CMAKE_MINIMUM_REQUIRED(VERSION 3.5)
 
+PROJECT(tntcxx)
+
 IF(NOT CMAKE_BUILD_TYPE)
     SET(CMAKE_BUILD_TYPE Debug)
 ENDIF()