mavlink_types.h: fix typo introduced in d41fe510d8cd

because this condition doesn't short circuit as expected when _MSC_VER is not defined, I get

    src/mavlink/c_library/mavlink_types.h:5:27: error: "_MSC_VER" is not defined [-Werror=undef]
     #if (defined _MSC_VER) & (_MSC_VER < 1800)
                           ^