Make Malloc.h self-contained again
Make Malloc.h self-contained again

Summary: D3743475 adds to `Malloc.h` a dependency on another folly header, which breaks it when used in stand-alone mode. This diff moves the include to the right `#ifdef` section.

Reviewed By: Gownta

Differential Revision: D3757819

fbshipit-source-id: 71664ca6a3a47b6e4449a4ef603fedf052c5df3b

facebook/folly,rklabs/folly,Orvid/folly,facebook/folly,floxard/folly,Orvid/folly,Orvid/folly,rklabs/folly,Orvid/folly,floxard/folly,rklabs/folly,facebook/folly,Orvid/folly,facebook/folly,charsyam/folly,floxard/folly,rklabs/folly,charsyam/folly,facebook/folly,floxard/folly,charsyam/folly,floxard/folly,charsyam/folly,rklabs/folly