build: Fix introspection on Visual Studio

Hi,

The current Meson build files could not properly support building introspection files for gsettings-desktop-schemas with Visual Studio builds because Visual Studio requires at least an object file to be linked in order to build a library or executable, which is in turn required by Meson to generate any introspection files. This attempts to fix this by generating a dummy .c file that we use to build the dummy library when we are building on Visual Studio.

With blessings, thank you!