Change std::to_string call to c10::to_string
Summary: I'm using this code in an internal Android build, and std::to_string doesn't work in our internal Android builds yet.
Test Plan: Internal build.
Reviewed By: ljk53
Differential Revision: D20234221
fbshipit-source-id: 8fd61235bf9b487e07a1459c452830e732c7afb0
