ibstdc++: Shut up -Wattribute-alias warning [PR109694]

I've followed what other files do, using attribute alias with not really
matching function type (after all, it isn't really possible when it is a
constructor), but seems I've missed it warns:
../../../../../libstdc++-v3/src/c++98/ios_init.cc:203:8: warning: ‘void std::ios_base_library_init()’ alias between functions of incompatible types ‘void()’ and ‘void
+(std::ios_base::Init::)()’ [-Wattribute-alias=]
  203 |   void ios_base_library_init (void)
      |        ^~~~~~~~~~~~~~~~~~~~~
../../../../../libstdc++-v3/src/c++98/ios_init.cc:78:3: note: aliased declaration here
   78 |   ios_base::Init::Init()
      |   ^~~~~~~~
The PR talks about clang++ warning there (which I think isn't really
supported, libstdc++ sources ought to be built by GCC), but it warns
when built with GCC too.

The following patch fixes it by doing what other libstdc++ sources do in
those cases.

2023-05-02  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/109694
	* src/c++98/ios_init.cc: Add #pragma GCC diagnostic ignored for
	-Wattribute-alias.

(cherry picked from commit 87de39e4)