Fix C declaration of dir functions and types
Fix C declaration of dir functions and types

This patch fixes the declaration of the DIR type and related functions
so that they can be called from C code. This is necessary when enabling
functionality that uses the filesystem in mbed TLS.

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os