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

Makefile

@@ -49,7 +49,7 @@ else
 	PLATFORM_CCFLAGS += $(JEMALLOC_INCLUDE) -DHAVE_JEMALLOC
 endif
 
-WARNING_FLAGS = -Wall -Werror
+WARNING_FLAGS = -Wall -Werror -Wsign-compare
 CFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT)
 CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual