sys/cpp11-compat: Add braces around mutex initializer

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.

sys/cpp11-compat/include/riot/mutex.hpp

@@ -44,7 +44,7 @@ class mutex {
  public:
   using native_handle_type = mutex_t*;
 
-  inline constexpr mutex() noexcept : m_mtx{0} {}
+  inline constexpr mutex() noexcept : m_mtx{{0}} {}
   ~mutex();
 
   void lock();