Add pkg-config to charmcraft.yaml for binary builds

Depending on the build platform (e.g. s390x, ppc64el), sometimes
pkg-config is required, and sometimes it isn't.  Add it to make sure it
is available depending on the availability of binary wheels for
architectures.

Closes-Bug: #2023777
Change-Id: I3cdcea3a6bcadd38680a30553649dc6ed5202dd5

charmcraft.yaml

@@ -13,6 +13,7 @@ parts:
       - libssl-dev
       - rustc
       - cargo
+      - pkg-config
     source: src/
     build-snaps:
       - charm/latest/edge

tox.ini

@@ -42,6 +42,7 @@ basepython = python3
 # `tox -e build` always performs a clean, repeatable build.
 # For faster rebuilds during development,
 # directly run `charmcraft -v pack && ./rename.sh`.
+deps =
 commands =
     charmcraft clean
     charmcraft -v pack