graphics/libheif: unbreak after 17339df99b0c

libheif/plugins/encoder_svt.cc:850:37: error: assigning to 'PredStructure' from \
incompatible type 'int'

850 | 	svt_config.pred_structure = 2; // RANDOM_ACCESS
    |                                     ^

Reported by: Tatsuki Makino, mfechner
Regressed by: https://gitlab.com/AOMediaCodec/SVT-AV1/-/commit/3b5e5e8d4dda
Obtained from: upstream (open PR)