Fix build issue with string.h and linux/string.h

This commit:
commit 338cc03846
Author: Wolfgang Denk <wd@denx.de>
Date:   Fri Jun 6 14:28:14 2008 +0200

    tools/mkimage: fix compiler warnings on some systems.

Broke building on some systems, because the host's string.h was interfering
with u-boot's linux/string.h.  It doesn't look like we need the u-boot one if
we're building for the host, so now we only include when building inside
u-boot.

Signed-off-by: Andy Fleming <afleming@freescale.com>

lib_generic/md5.c

@@ -27,12 +27,12 @@
 
 #ifndef USE_HOSTCC
 #include <common.h>
+#include <linux/string.h>
 #else
 #include <string.h>
 #endif /* USE_HOSTCC */
 #include <watchdog.h>
 #include <linux/types.h>
-#include <linux/string.h>
 #include <u-boot/md5.h>
 
 static void

lib_generic/sha1.c

@@ -31,11 +31,11 @@
 
 #ifndef USE_HOSTCC
 #include <common.h>
+#include <linux/string.h>
 #else
 #include <string.h>
 #endif /* USE_HOSTCC */
 #include <watchdog.h>
-#include <linux/string.h>
 #include "sha1.h"
 
 /*