multimedia/av1an: unbreak on aarch64/armv[67]/powerpc*

error[E0308]: mismatched types

--> cargo-crates/ffmpeg-next-4.4.0/src/util/error.rs:205:7
 |

205 | [[0_i8; AV_ERROR_MAX_STRING_SIZE]; 27];

|       ^^^^ expected `u8`, found `i8`
|

help: change the type of the numeric literal from i8 to u8

|

205 | [[0_u8; AV_ERROR_MAX_STRING_SIZE]; 27];

|       ~~~~

Reported by: pkg-fallout