be explicit about what we're passing to Endian::big

This prevents a linker error on OSX:
Undefined symbols for architecture x86_64:
  "folly::detail::EndianIntBase<unsigned long>::swap(unsigned long)",
  referenced from:
        __GLOBAL__sub_I_MacAddress.cpp in libfolly.a(MacAddress.cpp.o)
	ld: symbol(s) not found for architecture x86_64