trivial: set up win32 build not to take GPG by default
trivial: set up win32 build not to take GPG by default

Unfortunately you can't detect the meson option:
```
../data/pki/meson.build:4:4: ERROR: Having a colon in option name is forbidden, projects are not allowed to directly access options of other subprojects.
```
So hardcode in top level meson.build that Windows doesn't take GPG.

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd