Fix clang build error in NeighborCache.h
Fix clang build error in NeighborCache.h

Summary: Add template keyword to avoid ambiguity.

Reviewed By: aeckert

Differential Revision: D3031112

fb-gh-sync-id: 45ebc93b503df77c5345459812e063e082089deb
fbshipit-source-id: 45ebc93b503df77c5345459812e063e082089deb

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