Makefile: Run go vet as part of precheck
Makefile: Run go vet as part of precheck

This was only being caught by certain test targets, but usually we lint
things in precheck stage. Move 'go vet' invocation into precheck so that
it's actually caught in local builds in most cases.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@covalent.io>

tgraf/cilium,scanf/cilium,michi-covalent/cilium,eloycoto/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,eloycoto/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,tklauser/cilium,tgraf/cilium,scanf/cilium,cilium/cilium,eloycoto/cilium,michi-covalent/cilium,scanf/cilium,cilium/cilium,eloycoto/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,eloycoto/cilium,michi-covalent/cilium,scanf/cilium,cilium-team/cilium,scanf/cilium,eloycoto/cilium,tgraf/cilium,cilium/cilium,cilium-team/cilium,eloycoto/cilium