Only have -pc/-pk apply to syscall rules (#495)

Currently, -pc/-pk results in extra fields added to the output field of
all rules. They should only be added for syscall rules.

userspace/engine/lua/rule_loader.lua

@@ -493,6 +493,7 @@ function load_rules(sinsp_lua_parser,
 	 -- If the format string contains %container.info, replace it
 	 -- with extra. Otherwise, add extra onto the end of the format
 	 -- string.
+	 if v['source'] == "syscall" then
 	    if string.find(v['output'], "%container.info", nil, true) ~= nil then
 
 	       -- There may not be any extra, or we're not supposed
@@ -513,6 +514,7 @@ function load_rules(sinsp_lua_parser,
 		  v['output'] = v['output'].." "..extra
 	       end
 	    end
+	 end
 
 	 -- Ensure that the output field is properly formatted by
 	 -- creating a formatter from it. Any error will be thrown