parser: filename can contain "\" separator on Windows
Fix parsing of Windows filename by using proper GLib API instead of doing manually.
Meson had a hack to avoid passing Windows path separator since forever, but we would like to get ride of it eventually. Removing that hack in Meson exposes this bug in g-ir-scanner. Running C:\Users\Collabora\xclaesse\gstreamer\builddir\subprojects\gobject-introspection-1.74.0\tools\g-ir-compiler.exe -o subprojects\gobject-introspection-1.74.0\gir\DBusGLib-1.0.typelib ..\subprojects\gobject-introspection-1.74.0\gir\DBusGLib-1.0.gir --includedir C:\Users\Collabora\xclaesse\gstreamer\builddir\subprojects\gobject-introspection-1.74.0\gir --includedir C:\Users\Collabora\xclaesse\gstreamer\subprojects\gobject-introspection-1.74.0\gir fails with
..\subprojects\gobject-introspection-1.74.0\gir\DBusGLib-1.0.gir:11:1: error: <namespace/> name element 'DBusGLib' doesn't match file name '..\subprojects\gobject'
error parsing file ..\subprojects\gobject-introspection-1.74.0\gir\DBusGLib-1.0.gir: <namespace/> name element 'DBusGLib' doesn't match file name '..\subprojects\gobject'