makefile: run go mod tidy during make generate

We have multiple go.mod files, this does not seem to agree with operator-sdk.
operator-sdk will generate files and call `make generate`. However, that will then
fail because we didn't call `go mod tidy` for api/ directory.

Note that compared to sriov-network-operator, in dpu-operator we have
several go.mod files. That does not seem to agree with operator-sdk, or
at least, something is not hooked up right.

Hack `make generate` to call `go mod tidy` first.

Otherwise:

  $ git checkout -B C origin/main && \
    git reset --hard HEAD && \
    git clean -fdx && \
    make operator-sdk && \
    ./bin/operator-sdk create webhook --group config --version v1 --kind DpuOperatorConfig --defaulting --programmatic-validation
  branch 'C' set up to track 'origin/main'.
  Reset branch 'C'
  Your branch is up to date with 'origin/main'.
  HEAD is now at e58a9c9 Merge pull request #194 from thom311/th/hack-scripts-cleanup
  INFO[0000] Writing kustomize manifests for you to edit...
  ERRO[0000] Unable to find the target #- path: manager_webhook_patch.yaml to uncomment in the file config/default/kustomization.yaml.
  INFO[0000] Writing scaffold for you to edit...
  INFO[0000] api/v1/dpuoperatorconfig_webhook.go
  INFO[0000] api/v1/dpuoperatorconfig_webhook_test.go
  INFO[0000] api/v1/webhook_suite_test.go
  INFO[0000] Update dependencies:
  $ go mod tidy
  INFO[0000] Running make:
  $ make generate
  test -s /data/src/dpu-operator/bin/controller-gen && /data/src/dpu-operator/bin/controller-gen --version | grep -q v0.15.0 || \
  GOBIN=/data/src/dpu-operator/bin GOFLAGS='' go install sigs.k8s.io/controller-tools/cmd/[email protected]
  GOFLAGS='' /data/src/dpu-operator/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
  Error: load packages in root "/data/src/dpu-operator/api": err: exit status 1: stderr: go: updates to go.mod needed; to update it:
          go mod tidy

Makefile

@@ -143,7 +143,14 @@ prow-ci-manifests-check: manifests
 		exit 1; \
 	fi
 
+.PHONY: vendor
+vendor:
+	for d in . dpu-api api tools ; do \
+		(cd $$d; go mod tidy) || exit $$? ; \
+	done
+
 .PHONY: generate
+generate: vendor
 generate: controller-gen ## Generate code containing DeepCopy, DeepCopyInto, and DeepCopyObject method implementations.
 	GOFLAGS='' $(CONTROLLER_GEN) object:headerFile="hack/boilerplate.go.txt" paths="./..."