loader: userboot: provide a getsecs() implementation

We don't need it for userboot, but it avoids issues with BIND_NOW, so
just provide it.  time(3) isn't defined but ends up being provided by
libc linked into the host process, which is generally fine.

PR:	262920
Reviewed by:	imp, jhb
MFC after:	3 days
Diferential Revision:	https://reviews.freebsd.org/D34758

stand/userboot/userboot/main.c

@@ -70,6 +70,18 @@ delay(int usec)
 	CALLBACK(delay, usec);
 }
 
+time_t
+getsecs(void)
+{
+
+	/*
+	 * userboot can't do netboot, so this implementation isn't strictly
+	 * required.  Defining it avoids issues with BIND_NOW, and it doesn't
+	 * hurt to do it.
+	 */
+	return (time(NULL));
+}
+
 void
 exit(int v)
 {