GTK Inspector broken on Windows

GTK inspector can not be used anymore on a Windows 10 machine with MSYS2's mingw-w64 packages.

Calling GTK_DEBUG=interactive gtk3-demo

• works as expected with mingw-w64-x86_64-gtk3-3.24.8-1
• does not work with later versions, i.e. inspector window does not open
  (tested with mingw-w64-x86_64-gtk3-3.24.9-3, mingw-w64-x86_64-gtk3-3.24.9-4 and mingw-w64-x86_64-gtk3-3.24.10-1)