metadata: vorbis: fix warning when building sim on aarch64 linux
b64_codes is unsigned on some systems unless explicitly
declared as signed.
"b64_codes[index] < 0 comparison is always false due
to limited range of data type [-Werror=type-limits]"
Introduced in 849e8ab
Change-Id: I6a86e898c2ed8995f7346060f9b9b58ea6ab1e95