add changelog

changelog/unreleased/build-all-targets.md

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+Enhancement: Add new build targets
+
+Make build target `build` used to build a binary twice, the second occurrence having symbols for debugging. We split this step in two and added `build-all` and `build-debug` targets.
+
+- `build-all` now behaves as the previous `build` target, it will generate 2 binaries, one for debug.
+- `build-debug` will build a single binary for debugging.
+
+https://github.com/owncloud/ocis/pull/1824