Fix clang build

Summary:
GTEST dont compile under clang when -Werror=missing-field-initializers is set
revert this change

Test Plan:
USE_CLANG=1 make check
make check

Reviewers: rven

Reviewed By: rven

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D52455

Makefile

@@ -192,7 +192,7 @@ ifndef DISABLE_WARNING_AS_ERROR
 endif
 
 CFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT)
-CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual -Wnon-virtual-dtor -Werror=missing-field-initializers
+CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers
 
 LDFLAGS += $(PLATFORM_LDFLAGS)