win32: Fix meson names for defines

Those have been wrong forever, but nobody cares because they get to the
values we want anyway.
5 jobs for win32-enable-werror in 13 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #304729
fedora-x86_64

00:04:35

passed #304730
win32
msys2-mingw32

00:13:57

 
  Flatpak
manual #304731
allowed to fail manual
flatpak-manual:demo
manual #304733
allowed to fail manual
flatpak-manual:icon-browser
manual #304732
allowed to fail manual
flatpak-manual:widget-factory