ci: check out simplexmq's submodules in the smp-server clone [claude-test]

The clone compiled all of simplexmq, then failed on "does not exist: cbits/blst/src/server.c" (run 38776361371): cbits/blst and cbits/libbbs are git submodules, which cabal's own clones check out and this one did not.

scripts/ci/ios-sim/build-backend.sh

@@ -34,7 +34,9 @@ cabal build -j $targets
 simplexmq=$(mktemp -d)/simplexmq
 git clone --quiet --filter=blob:none https://github.com/simplex-chat/simplexmq.git "$simplexmq"
 git -C "$simplexmq" checkout --quiet "$(awk '/simplexmq.git/ {getline; print $2; exit}' cabal.project)"
+git -C "$simplexmq" submodule update --init --quiet
 git -C "$simplexmq" log -1 --format='simplexmq %H %s'
+git -C "$simplexmq" submodule status
 cp cabal.project.local "$simplexmq/"
 (cd "$simplexmq" && cabal build -j exe:smp-server)