Fix include/wchar.h for C++

When trying to compile regression tests that use
C++ and the threads header you get this failure:

In file included from /usr/include/c++/5.3.1/cwchar:44:0,
from /usr/include/c++/5.3.1/bits/postypes.h:40,
from /usr/include/c++/5.3.1/bits/char_traits.h:40,
from /usr/include/c++/5.3.1/string:40,
from /usr/include/c++/5.3.1/stdexcept:39,
from /usr/include/c++/5.3.1/array:38,
from /usr/include/c++/5.3.1/tuple:39,
from /usr/include/c++/5.3.1/functional:55,
from /usr/include/c++/5.3.1/thread:39,
from tst-thread-quick_exit.cc:19:
../include/wchar.h:105:23: error: invalid conversion from ‘wchar_t*
(*)(wchar_t*, wchar_t, size_t) throw () {aka wchar_t* (*)(wchar_t*,
wchar_t, long unsigned int) throw ()}’ to ‘int’ [-fpermissive]
extern typeof (wmemset) __wmemset;
^
../include/wchar.h:105:25: error: expected ‘,’ or ‘;’ before ‘__wmemset’
extern typeof (wmemset) __wmemset;
^

The simplest fix for C++ is to avoid the use of
typeof and just declare the prototype as expected.

No regressions on x86_64. Committed as obvious.
The include/wchar.h header is only for internal
build uses and therefore is not ever seen by any
external users and needs no bug #.