gconv.h: fix build with GCC 6.3 or later

  gconv.h is using a flex array to define the __gconv_info member in an
invalid way, causing the future GCC 6.3 (ie the gcc-6-branch) or the
future GCC 7 (ie the master branch) to issue an error:

| In file included from ../include/gconv.h:1:0,
|                  from ../sysdeps/unix/sysv/linux/_G_config.h:32,
|                  from ../libio/libio.h:31,
|                  from ../include/libio.h:4,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../iconv/gconv.h:142:50: error: flexible array member '__gconv_info::__data' not at end of 'struct _IO_codecvt'
| In file included from ../include/libio.h:4:0,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../libio/libio.h:211:14: note: next member '_G_iconv_t _IO_codecvt::__cd_out' declared here
| ../libio/libio.h:187:8: note: in the definition of 'struct _IO_codecvt'
| In file included from ../include/gconv.h:1:0,
|                  from ../sysdeps/unix/sysv/linux/_G_config.h:32,
|                  from ../libio/libio.h:31,
|                  from ../include/libio.h:4,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../iconv/gconv.h:142:50: error: flexible array member '__gconv_info::__data' not at end of 'struct _IO_wide_data'
| In file included from ../include/libio.h:4:0,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../libio/libio.h:211:14: note: next member '_G_iconv_t _IO_codecvt::__cd_out' declared here
| ../libio/libio.h:215:8: note: in the definition of 'struct _IO_wide_data'

This is basically a revert to the code from 15 years ago. More details
are available in the GCC bug:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78039

Changelog:
	* iconv/gconv.h (__gconv_info): Define __data element using a
	zero-length array.
---
 ChangeLog     | 5 +++++
 iconv/gconv.h | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)
  

On Wed, 19 Oct 2016, Aurelien Jarno wrote:

> 	* iconv/gconv.h (__gconv_info): Define __data element using a
> 	zero-length array.

OK.
  

On 2016-11-05 21:52, Joseph Myers wrote:
> On Wed, 19 Oct 2016, Aurelien Jarno wrote:
> 
> > 	* iconv/gconv.h (__gconv_info): Define __data element using a
> > 	zero-length array.
> 
> OK.

Thanks for the review. Note that in the meantime GCC upstream reverted
the change from the GCC 6 branch, but it's still there in the trunk. I
have adjusted the commit message in consequence before committing it.