fix GFLAGS conflict
fix GFLAGS conflict

Summary: we have linkscan_interval_us defined both in BcmSwitch.cpp and lld_tool.cpp; keep it only in BcmSwitch.cpp

Reviewed By: allwync

Differential Revision: D4233709

fbshipit-source-id: 51c6831efde8ddfc6140e281c4e3b8645adc3ed9

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