Fix the build of MallctlHelperTest without JEMalloc
Fix the build of MallctlHelperTest without JEMalloc

Summary:
`kDecayCmd` was guarded such that it wasn't being defined at all if JEMalloc wasn't available.

Closes https://github.com/facebook/folly/issues/608

Reviewed By: interwq

Differential Revision: D5193721

fbshipit-source-id: 1a2666de70e90eac1e9f2e130d0452b01b3239fa

rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly,Orvid/folly,Orvid/folly,Orvid/folly,rklabs/folly,facebook/folly,facebook/folly