Fixes Clang warning/error:
In file included from .../riot/sys/cpp11-compat/thread.cpp:26:
In file included from .../riot/sys/cpp11-compat/include/riot/thread.hpp:39:
.../riot/sys/cpp11-compat/include/riot/mutex.hpp:47:45: error:
suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
inline constexpr mutex() noexcept : m_mtx{0} {}
^
{}
1 error generated.
These headers do not provide full stl functionality,
but a small subset:
* thread and this_thread
* condition_variable (some timed functions are missing)
* mutex, lock_guard and unique_lock
