build: Let make rebuild the bundle on PO changes

[martinpitt]

Fixes #19406

---

Before:

$ touch po/de.po; make
make  all-am
make[1]: Entering directory '/var/home/martin/upstream/cockpit/main'
make[1]: Leaving directory '/var/home/martin/upstream/cockpit/main'

Now:

make  all-am
make[1]: Entering directory '/var/home/martin/upstream/cockpit/main'
  BUNDLE   dist
12:03:23: Build finished
  COPY     dist/static/fonts/RedHatText-Regular.woff2 → doc/guide/html/
  COPY     dist/static/fonts/RedHatText-Medium.woff2 → doc/guide/html/
  GEN      doc/guide/html/index.html
make[1]: Leaving directory '/var/home/martin/upstream/cockpit/main'

[jelly]

Thanks!