Apparently OSS build needs <folly/MapUtil.h>
Apparently OSS build needs <folly/MapUtil.h>

Summary:
The admin distance code compiled in production, but broke the open
source build.  Honestly not sure why it compiles in production, but
adding an explicit include for folly/MapUtil fixes the build.

Differential Revision: D5173917

fbshipit-source-id: c4adedc958ef42a9e2ddccc1627c9e975f71ad8e

peterlei/fboss,peterlei/fboss,peterlei/fboss,peterlei/fboss