Ensure `wint_t' is defined before use in include/stdio.h

Ensure `wint_t' is defined before use in include/stdio.h

While trying to get nptl/tst-initializers1.c to include the test skeleton, I
came across a couple of speed bumps. Firstly: after making the appropriate
changes to the test, running `make check' led to this error:

> In file included from ../malloc/malloc.h:24:0,
..
>                  from tst-initializers1.c:60:
> ../include/stdio.h:111:1: error: unknown type name `wint_t'
>  extern wint_t __getwc_unlocked (FILE *__fp);

So, `wint_t' is used before being defined. Question: Why did test-skeleton.c
not cause this error in any of the other tests that include it?

Anyway, I noticed include/stdio.h includes stddef.h, which in turn defines
`wint_t', but only if `__need_wint_t' is defined. So I put in a
`#define __need_wint_t' before the include to get rid of the error. Is that
the correct fix?

A subsequent `make && make check' led to this second error:

>                  from tst-initializers1-c89.c:1:
> ../test-skeleton.c: In function `main':
> ../test-skeleton.c:356:11: error: `for' loop initial declarations are only
>  allowed in C99 mode
>            for (struct temp_name_list *n = temp_name_list;

Although there seem to be several other C89 no-noes in test-skeleton.c, I
needed only to fix this specific one for gcc-4.8.3 to stop complaining.