newlib: libc: reent.h: remove unnecessary parentheses

The compiler warns the double parentheses are unnecessary in some
target, and cause fail cases when doing some testcases in regression.

gcc/testsuite/g++.dg/warn/Wstringop-overflow-6.C

Remove the unnecessary parentheses will fix it. See more details in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85775

Same like in commit 0542583129,
Author: Maxim Blinov <maxim.blinov@embecosm.com>
Date:   Thu Jul 22 22:41:42 2021 +0100

     Remove unneccesary parenthesis around declarator

Thanks for Sebastian Huber's remind!

newlib/libc/include/sys/reent.h

@@ -421,7 +421,7 @@ struct _reent
   char *_asctime_buf;
 
   /* signal info */
-  void (**(_sig_func))(int);
+  void (** _sig_func)(int);
 
 #ifdef _REENT_BACKWARD_BINARY_COMPAT
   struct _atexit *_reserved_6;