lua: CMakeLists: drop redundant cmake_minimum_required

lua: CMakeLists: drop redundant cmake_minimum_required

The Lua module depends on the parent uci project and can't be build
independently.

Drop redundant cmake_minimum_required from lua/CMakeLists.txt and
inherit the version from the parent CMake project to keep version
consistency.

Signed-off-by: David Härdeman <[email protected]>
[ improve commit description ]
Link: https://github.com/openwrt/uci/pull/7
Signed-off-by: Christian Marangi <[email protected]>