Mark app/concerns as autoload

To work with metasploit-concern 0.4.0 prerelease not deriving
app/concerns from root and to ensure it is does not inherit eager_load
from app.

Author: limhoff-r7

lib/metasploit/framework/common_engine.rb

@@ -32,6 +32,7 @@ module Metasploit::Framework::CommonEngine
     end
 
     config.root = Msf::Config::install_root
+    config.paths.add 'app/concerns', autoload: true
     config.paths.add 'data/meterpreter', glob: '**/ext_*'
     config.paths.add 'modules'