util/util/cpuinfo-riscv.c: fix riscv64 build on musl libc

util/util/cpuinfo-riscv.c: fix riscv64 build on musl libc

build fails on musl libc (alpine linux) with this error:

../util/cpuinfo-riscv.c: In function 'cpuinfo_init':
../util/cpuinfo-riscv.c:63:21: error: '__NR_riscv_hwprobe' undeclared (first use in this function); did you mean 'riscv_hwprobe'?
   63 |         if (syscall(__NR_riscv_hwprobe, &pair, 1, 0, NULL, 0) == 0
      |                     ^~~~~~~~~~~~~~~~~~
      |                     riscv_hwprobe
../util/cpuinfo-riscv.c:63:21: note: each undeclared identifier is reported only once for each function it appears in
ninja: subcommand failed

add '#include "asm/unistd.h"' to util/cpuinfo-riscv.c fixes build

Signed-off-by: Milan P. Stanić <mps@arvanta.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240905150702.2484-1-mps@arvanta.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>