tools/mkimage: fix compiler warnings on some systems.

Signed-off-by: Wolfgang Denk <wd@denx.de>

lib_generic/md5.c

@@ -27,6 +27,8 @@
 
 #ifndef USE_HOSTCC
 #include <common.h>
+#else
+#include <string.h>
 #endif /* USE_HOSTCC */
 #include <watchdog.h>
 #include <linux/types.h>

lib_generic/sha1.c

@@ -31,6 +31,8 @@
 
 #ifndef USE_HOSTCC
 #include <common.h>
+#else
+#include <string.h>
 #endif /* USE_HOSTCC */
 #include <watchdog.h>
 #include <linux/string.h>