Fix Clang warnings in DRM code

[ChrisThrasher]

Subject of the issue

/home/thrasher/Projects/sfml/src/SFML/Window/DRM/InputImplUDev.cpp:415:36: error: implicit conversion changes signedness: 'sf::Keyboard::Key' to 'std::vector<bool, type-parameter-0-0>::size_type' (aka 'unsigned long') [-Werror,-Wsign-conversion]
                            keyMap[kb] = inputEvent.value;
                            ~~~~~~ ^~
/home/thrasher/Projects/sfml/src/SFML/Window/DRM/InputImplUDev.cpp:496:39: error: use of old-style cast [-Werror,-Wold-style-cast]
        newTerminalConfig.c_lflag &= ~(tcflag_t)ICANON;
                                      ^         ~~~~~~
/home/thrasher/Projects/sfml/src/SFML/Window/DRM/InputImplUDev.cpp:567:19: error: implicit conversion changes signedness: 'Keyboard::Key' to 'std::vector<bool, type-parameter-0-0>::size_type' (aka 'unsigned long') [-Werror,-Wsign-conversion]
    return keyMap[key];
           ~~~~~~ ^~~
/home/thrasher/Projects/sfml/src/SFML/Window/DRM/InputImplUDev.cpp:695:35: error: use of old-style cast [-Werror,-Wold-style-cast]
    newTerminalConfig.c_lflag &= ~(tcflag_t)ECHO;         // disable console feedback
                                  ^         ~~~~
/home/thrasher/Projects/sfml/src/SFML/Window/DRM/InputImplUDev.cpp:696:35: error: use of old-style cast [-Werror,-Wold-style-cast]
    newTerminalConfig.c_lflag &= ~(tcflag_t)ISIG;         // disable signals
                                  ^         ~~~~
5 errors generated.

Clang generates some warnings in the DRM code that GCC does not. It's just a matter of fixing some casts. The warnings are present in 2.6.x and master so let's fix this in 2.6.x so both branches can benefit from it.

[ChrisThrasher]

Fixed in #2297