lib/rsa/rsa-sig.c: compile on OS X

Interfaces exposed by error.h seems not to be used in rsa-sig.c, remove it.
This also fixes an compile error on OS X:

---8<---
u-boot/lib/rsa/rsa-sign.c:23:19: error: error.h: No such file or directory
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>

lib/rsa/rsa-sign.c

@@ -20,7 +20,6 @@
 #include "mkimage.h"
 #include <stdio.h>
 #include <string.h>
-#include <error.h>
 #include <image.h>
 #include <time.h>
 #include <openssl/rsa.h>