xxd reads standard input when given no file operand, so the - is
redundant. It also breaks builds that provide xxd through a partial
reimplementation such as OpenWrt's scripts/xxdi.pl, which treats - as a
file name and exits with
xxdi.pl: Unable to open -: No such file or directory
xml_plist_codegen.sh does not check for that failure, so the generated
plists/get_info_response.c ends up with an empty byte array and AirPlay 2
answers GET /info with nothing.
