d/rules: Fix LTO warning.

Nathan Teodosiorequested to merge
nteodosio/restic:master into master

Removes

dh_auto_build: warning: LTO optimize is enable in buildflags. But cgo doesn't support it. LTO flags will be stripped in cgo

As requested in https://bugs.launchpad.net/ubuntu/+source/restic/+bug/2134530/comments/1.

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