jehanne: protect sys/dirent.h functions

newlib/libc/sys/jehanne/sys/dirent.h

@@ -41,6 +41,9 @@ typedef struct {
 #define dirent64		dirent
 #endif
 
+#ifndef _DIRENT_H_
+/* Newlib's dirent.h already defines these */
+
 DIR *opendir(const char *);
 struct dirent *readdir(DIR *);
 int readdir_r(DIR *__restrict, struct dirent *__restrict,
@@ -50,6 +53,8 @@ int dirfd(DIR *);
 DIR *fdopendir(int);
 int closedir(DIR *);
 
+#endif
+
 #ifndef _POSIX_SOURCE
 long telldir (DIR *);
 void seekdir (DIR *, off_t loc);