riscv: define NR_syscalls in unistd.h

This macro is used by kernel/trace/{trace.h,trace_syscalls.c} if we
have CONFIG_FTRACE_SYSCALLS enabled.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Fixes: b78002b395b4 ("riscv: add HAVE_SYSCALL_TRACEPOINTS to Kconfig")
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>