fix build command
fix build command

`make` alone doesn't work to build alertmanager, throws a pile of errors instead here. `make build` works perfectly fine, though.
brancz/alertmanager,novilabs/alertmanager,Kellel/alertmanager,tinytub/alertmanager,sarathsp06/alertmanager,tyrken/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,brunoqc/alertmanager,tbouvet/alertmanager,brancz/alertmanager,GoogleCloudPlatform/alertmanager,novilabs/alertmanager,prometheus/alertmanager,sarathsp06/alertmanager,GoogleCloudPlatform/alertmanager,novilabs/alertmanager,GoogleCloudPlatform/alertmanager,brunoqc/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,tbouvet/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,brunoqc/alertmanager,tinytub/alertmanager,brunoqc/alertmanager,Aloomaio/alertmanager,Aloomaio/alertmanager,ericgerling/alertmanager,ericgerling/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,brancz/alertmanager,tbouvet/alertmanager,novilabs/alertmanager,brancz/alertmanager,Kellel/alertmanager,sarathsp06/alertmanager,tyrken/alertmanager,ericgerling/alertmanager,tinytub/alertmanager