games/libretro-bluemsx: fix build on powerpc64le

Src/SoundChips/OpenMsxYM2413.cpp:331:10: error: constant expression evaluates to -3 which cannot be narrowed to type 'char' [-Wc++11-narrowing]

7, 3, 0,-3,-7,-3, 0, 3,

Since it should also fix build on other architectures with unsigned char, unbreak on aarch64 and riscv64 (untested).