Remove unneccesary parenthesis around declarator

riscv64-unknown-elf-g++-11.1.0 regression suite reports the following
failures for

$ make check-gcc-c++ RUNTESTFLAGS='dg.exp=Wstringop-overflow-6.C'

```
FAIL: g++.dg/warn/Wstringop-overflow-6.C  -std=gnu++14 (test for excess errors)
FAIL: g++.dg/warn/Wstringop-overflow-6.C  -std=gnu++17 (test for excess errors)
FAIL: g++.dg/warn/Wstringop-overflow-6.C  -std=gnu++2a (test for excess errors)
UNSUPPORTED: g++.dg/warn/Wstringop-overflow-6.C  -std=gnu++98
```

The "excess errors" being

```
output is In file included from /home/maxim/prj/riscv-upstream/install/riscv64-unknown-elf/include/wchar.h:6,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/cwchar:44,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/bits/postypes.h:40,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/iosfwd:40,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/ios:38,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/ostream:38,
                 from /home/maxim/prj/riscv-upstream/build/gcc-stage2/riscv64-unknown-elf/libstdc++-v3/include/iostream:39,
                 from /home/maxim/prj/riscv-upstream/gcc-11.1.0/gcc/testsuite/g++.dg/warn/Wstringop-overflow-6.C:6:
/home/maxim/prj/riscv-upstream/install/riscv64-unknown-elf/include/sys/reent.h:685:11: warning: unnecessary parentheses in declaration of '_sig_func' [-Wparentheses]
```

newlib/libc/include/sys/reent.h

@@ -682,7 +682,7 @@ struct _reent
 # endif
 
   /* signal info */
-  void (**(_sig_func))(int);
+  void (**_sig_func)(int);
 
   /* These are here last so that __FILE can grow without changing the offsets
      of the above members (on the off chance that future binary compatibility