build: add -e to fix make generate

[marioferh]

With new go versions this error apperas:

cluster-monitoring-operator git:(master) make generate                                                                                                     
Installing tools from hack/tools.go                                                                                                                           
tools.go:22:2: import "github.com/brancz/gojsontoyaml" is a program, not an importable package           

In order to fix id an -e is added to make generate. Related #2112

[openshift-ci]

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[openshift-ci]

@marioferh: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-agnostic-operator	5342dee	link	true	/test e2e-agnostic-operator
ci/prow/e2e-aws-ovn-single-node	5342dee	link	false	/test e2e-aws-ovn-single-node
ci/prow/e2e-aws-ovn-techpreview	5342dee	link	true	/test e2e-aws-ovn-techpreview
ci/prow/versions	5342dee	link	false	/test versions

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