cmake: add CXX_STANDARD_REQUIRED flag to ensure c++ version must be c…

…++26 (#9476)

As the [cmake documentation](https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD_REQUIRED.html) said, when we use the CMAKE_CXX_STANDARD without CXX_STANDARD_REQUIRED, then the standard version we set will be an optional value, and may decay back to older c++ version. So I add this configuration into cmakelist to ensure c++ version is c++26.

CMakeLists.txt

@@ -77,6 +77,7 @@ add_compile_definitions(HYPRLAND_VERSION="${HYPRLAND_VERSION}")
 include_directories(. "src/" "protocols/")
 
 set(CMAKE_CXX_STANDARD 26)
+set(CXX_STANDARD_REQUIRED ON)
 add_compile_options(
   -Wall
   -Wextra