Makefile: Fix govet target using go1.12
Makefile: Fix govet target using go1.12

See go docs for 1.12, https://golang.org/doc/go1.12#vet

```
go tool vet no longer supported
The go vet command has been rewritten to serve as the base for a range
of different source code analysis tools. See the
golang.org/x/tools/go/analysis package for details. A side-effect is
that go tool vet is no longer supported. External tools that use go tool
vet must be changed to use go vet. Using go vet instead of go tool vet
should work with all supported versions of Go.

As part of this change, the experimental -shadow option is no longer
available with go vet. Checking for variable shadowing may now be done
using

go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
go vet -vettool=$(which shadow)
```

vmware/govmomi,bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi