Fix builds against glib-2.75.x+ with -Dwarnings=fatal on Visual Studio

Hi,

This updates generate_wrap_init.pl.in to insert the following into the generated wrap_init.cc:

...
#ifdef _MSC_VER
#pragma warning ( disable : 4273)
#pragma warning ( push )

extern "C"
{
  <C-style prototypes of C library where C++ binding is to be generated against>
  ...
}

#ifdef _MSC_VER
#pragma warning ( pop )
#endif
...

This is necessary as glib-2.75.x or later used macros that decorated prototypes in its headers for shared/DLL builds which expand to __declspec(dllimport), which will cause C4273 warnings (inconsistent decoration of function prototypes with and without __declspec(dllimport)) to be raised. Since this warning is harmless for this section of code, I think it would be best to just disable it momentarily.

This will also be needed for the glibmm-2-66 branch.

With blessings, thank you!