lang/rust: Add manifest-rustfmt-preview to PLIST
This wasn't reported by a simple poudriere bulk, but a `poudriere
testport` revealed the problem:
====> Running Q/A tests (stage-qa)
Error: 'lib/rustlib/manifest-rustfmt-preview' is referring to /wrkdirs/usr/ports/lang/rust/work/stage
Warning: 'bin/rustfmt' is not stripped consider trying INSTALL_TARGET=install-strip or using ${STRIP_CMD}
...
===> Checking for items in STAGEDIR missing from pkg-plist
Error: Orphaned: bin/rustfmt
Error: Orphaned: lib/rustlib/manifest-rustfmt-previewPR: 225445
Reported by: jbeich@