Add -Wsign-compare to WARNING_FLAGS in Makefile

Summary:
Add -Wsign-compare to WARNING_FLAGS in Makefile as not all g++ compiler
include -Wsign-compare in -Wall when compiling '.h' file.

Test Plan: make -j32

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D20169