add benchmark plugin builder

mythril/laser/plugin/plugins/benchmark.py

@@ -1,5 +1,5 @@
 from mythril.laser.ethereum.svm import LaserEVM
-from mythril.laser.plugin import LaserPlugin
+from mythril.laser.plugin import LaserPlugin, PluginBuilder
 from time import time
 import matplotlib.pyplot as plt
 import logging
@@ -7,6 +7,13 @@ import logging
 log = logging.getLogger(__name__)
 
 
+class BenchmarkPluginBuilder(PluginBuilder):
+    plugin_name = "benchmark"
+
+    def __call__(self, *args, **kwargs):
+        return BenchmarkPlugin()
+
+
 # TODO: introduce dependency on coverage plugin
 class BenchmarkPlugin(LaserPlugin):
     """Benchmark Plugin