add guards to SDLBackend.h
add guards to SDLBackend.h


bdonlan/openc2e,ccdevnet/openc2e,crystalline/openc2e,bdonlan/openc2e,ccdevnet/openc2e,crystalline/openc2e,ccdevnet/openc2e,ccdevnet/openc2e,crystalline/openc2e,bdonlan/openc2e,crystalline/openc2e,ccdevnet/openc2e,crystalline/openc2e,bdonlan/openc2e,ccdevnet/openc2e