[cost] Add .PHONY in cost.make
[cost] Add .PHONY in cost.make

Reviewed By: jberdine

Differential Revision: D22002143

fbshipit-source-id: dd036d5d4

jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer