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.

Author: Andres AG

platform/mbed_retarget.h

@@ -50,15 +50,21 @@ typedef int mode_t;     ///< Mode for opening files
 #if __cplusplus
 namespace mbed { class Dir; }
 typedef mbed::Dir DIR;
+#else
+typedef struct Dir DIR;
+#endif
 
+#if __cplusplus
 extern "C" {
+#endif
     DIR *opendir(const char*);
     struct dirent *readdir(DIR *);
     int closedir(DIR*);
     void rewinddir(DIR*);
     long telldir(DIR*);
     void seekdir(DIR*, long);
     int mkdir(const char *name, mode_t n);
+#if __cplusplus
 };
 #endif