Warnings with ffmpeg 7.1

[sfan5]

In file included from ../libplacebo/src/include/libplacebo/utils/libav.h:293,
                 from ../libplacebo/src/tests/libav.c:2:
../libplacebo/src/include/libplacebo/utils/libav_internal.h: In function ‘pl_system_from_av’:
../libplacebo/src/include/libplacebo/utils/libav_internal.h:52:5: warning: enumeration value ‘AVCOL_SPC_IPT_C2’ not handled in switch [-Wswitch]
   52 |     switch (spc) {
      |     ^~~~~~
../libplacebo/src/include/libplacebo/utils/libav_internal.h:52:5: warning: enumeration value ‘AVCOL_SPC_YCGCO_RE’ not handled in switch [-Wswitch]
../libplacebo/src/include/libplacebo/utils/libav_internal.h:52:5: warning: enumeration value ‘AVCOL_SPC_YCGCO_RO’ not handled in switch [-Wswitch]