Fix #13839 (False positive: warning about readdir, and readdir_r is deprecated) (#7525)

Author: danmar

cfg/posix.cfg

@@ -4095,7 +4095,6 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
       <not-uninit/>
       <not-bool/>
     </arg>
-    <warn severity="portability">Non reentrant function 'readdir' called. For threadsafe applications it is recommended to use the reentrant replacement function 'readdir_r'.</warn>
   </function>
   <!-- int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result); -->
   <function name="readdir_r">

test/cfg/posix.c

@@ -991,7 +991,6 @@ void nullPointer(char *p, int fd, pthread_mutex_t mutex)
     mkdir(p, 0);
     getcwd(0, 0);
     // cppcheck-suppress nullPointer
-    // cppcheck-suppress readdirCalled
     readdir(0);
     // cppcheck-suppress nullPointer
     // cppcheck-suppress utimeCalled