fix lint error

Author: johzchen

apisix/balancer.lua

@@ -14,18 +14,20 @@
 -- See the License for the specific language governing permissions and
 -- limitations under the License.
 --
+local healthcheck
+local require     = require
 local roundrobin  = require("resty.roundrobin")
 local discovery   = require("apisix.discovery.init").discovery
 local resty_chash = require("resty.chash")
 local balancer    = require("ngx.balancer")
 local core        = require("apisix.core")
-local healthcheck = require("resty.healthcheck")
 local error       = error
 local str_char    = string.char
 local str_gsub    = string.gsub
 local pairs       = pairs
 local ipairs      = ipairs
 local tostring    = tostring
+
 local set_more_tries   = balancer.set_more_tries
 local get_last_failure = balancer.get_last_failure
 local set_timeouts     = balancer.set_timeouts
@@ -82,6 +84,9 @@ end
 
 
 local function create_checker(upstream, healthcheck_parent)
+    if healthcheck == nil then
+        healthcheck = require("resty.healthcheck")
+    end    
     local checker = healthcheck.new({
         name = "upstream#" .. healthcheck_parent.key,
         shm_name = "upstream-healthcheck",