mysys/my_thr_init.c : Avoid warnings of "unused variable" by extending a '#ifdef'.

mysys/my_thr_init.c:
  Enclose a variable declaration in the same '#ifdef' that also controls all access to the variable.
  This avoids warnings of "unused variable".

mysys/my_thr_init.c

@@ -403,9 +403,9 @@ const char *my_thread_name(void)
 
 static uint get_thread_lib(void)
 {
+#ifdef _CS_GNU_LIBPTHREAD_VERSION
   char buff[64];
     
-#ifdef _CS_GNU_LIBPTHREAD_VERSION
   confstr(_CS_GNU_LIBPTHREAD_VERSION, buff, sizeof(buff));
 
   if (!strncasecmp(buff, "NPTL", 4))