use '@{' / '@}', fixes compile errors:
*** '{' without macro. Before: 1:-'.'}
*** '}' without opening '{' before:

since:
907ac20a avformat/hlsenc: added HLS encryption
Signed-off-by: James Zern <jzern@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>