cmd/cat.c: Add missing include

This file references errno values and so needs to include
<linux/errno.h>.

Signed-off-by: Tom Rini <trini@konsulko.com>

cmd/cat.c

@@ -8,6 +8,7 @@
 #include <fs.h>
 #include <malloc.h>
 #include <mapmem.h>
+#include <linux/errno.h>
 
 static int do_cat(struct cmd_tbl *cmdtp, int flag, int argc,
 		  char *const argv[])