portability: Avoid void* arithmetics which is a GNU extension

Under GNU C, sizeof(void) = 1. This commit merely makes it explicit u8.

Pointer arithmetics over void types is:
 * A GNU C extension
 * Not supported by Clang
 * Illegal across all ISO C standards

See also: https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html



Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>