audio/libsidplay2: fix build on powerpc64

[pkubaj]

Summary

Doesn't build due to:
xsid.cpp:99:5: error: constant expression evaluates to 128 which cannot be narrowed to type 'int8_t' (aka 'signed char') [-Wc++11-narrowing]

char is by default signed on powerpc64.

EDIT: Actually, this was a mistake, char is unsigned (it's signed on amd64).

[linimon]

Mentor approval (builds on ppcports).

[pkubaj]

Builds now.