tools/nolibc: fix build warning in sys_mmap() when my_syscall6 is not defined

We return -ENOSYS when there's no syscall6() operation, but we must cast
it to void* to avoid a warning.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/sys.h

@@ -692,7 +692,7 @@ void *sys_mmap(void *addr, size_t length, int prot, int flags, int fd,
 {
 #ifndef my_syscall6
 	/* Function not implemented. */
-	return -ENOSYS;
+	return (void *)-ENOSYS;
 #else
 
 	int n;