Update Makefile

Add go get in makefile because we removed the vendor folder and need to download the dependencies first.

Makefile

@@ -13,7 +13,7 @@ LDFLAGS = -extldflags "-s -w -static" \
 		  -X github.com/prometheus/common/version.Branch=$(BRANCH)
 
 $(EXE): $(SRC)
-	CGO_ENABLED=0 GOOS=linux go build -a -ldflags '$(LDFLAGS)' -o $(EXE) .
+	CGO_ENABLED=0 GOOS=linux go get && go build -a -ldflags '$(LDFLAGS)' -o $(EXE) .
 
 fmt:
 	go fmt ./...