[snmp]: Add swss dependency for snmp service (#1034)

- snmp service needs swss to be ready to get the port counter map

Signed-off-by: Shu0T1an ChenG <[email protected]>

Author: Shuotian Cheng

files/build_templates/snmp.service.j2

@@ -1,7 +1,7 @@
 [Unit]
 Description=SNMP container
-Requires=database.service
-After=database.service
+Requires=database.service swss.service
+After=database.service swss.service
 
 [Service]
 User={{ sonicadmin_user }}