Before newlib 3.3.0, <dirent.h> bundled in newlib did not include any
function declarations. Instead, the file included the platform-
specific <sys/dirent.h>. This inclusion was inside a C++ guard block.
ESP-IDF provided sys/dirent.h inside newlib component, and this file
contained all the necessary function and structure declarations.
Since da418955f5,
common function declarations have been added to <dirent.h> in newlib.
However, the inclusion of sys/dirent.h has been moved out of the C++
guard block. However we didn't notice this change and did not update
sys/dirent.h in ESP-IDF newlib component to and the now-required
C++ guards there.
This commit adds the missing C++ guards to the platform-specific
sys/dirent.h.
The declarations of common dirent.h functions are now present both in
<dirent.h> (provided by newlib) and in sys/dirent.h (provided by IDF).
We keep the declarations in sys/dirent.h for compatibility, since some
ESP-IDF files and applications may include <sys/dirent.h> directly,
rather than <dirent.h>.
Closes https://github.com/espressif/esp-idf/issues/7204
