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>

arch/riscv/include/asm/unistd.h

@@ -19,3 +19,5 @@
 #define __ARCH_WANT_SYS_CLONE
 
 #include <uapi/asm/unistd.h>
+
+#define NR_syscalls (__NR_syscalls)