(maint) Pin ffi on Windows

Pin ffi on Windows due to pxp-agent tests failing.

Even though they are deprecation warnings, having output on stderr may
break things in unexpected ways.

See: chef/win32-service#70

configs/components/rubygem-ffi.rb

@@ -7,6 +7,9 @@
   # Windows versions of the FFI gem have custom filenames, so we overwite the
   # defaults that _base-rubygem provides here, just for Windows.
   if platform.is_windows?
+    # Pin ffi on Windows due to win32-service failures
+    # see: https://github.com/chef/win32-service/issues/70
+    pkg.version '1.9.25'
     # Vanagon's `pkg.mirror` is additive, and the _base_rubygem sets the
     # non-Windows gem as the first mirror, which is incorrect. We need to unset
     # the list of mirrors before adding the Windows-appropriate ones here: