encrypted-keys: IS_ERR need include/err.h

Fixes this build error:

security/keys/encrypted-keys/masterkey_trusted.c: In function 'request_trusted_key':
security/keys/encrypted-keys/masterkey_trusted.c:35:2: error: implicit declaration of function 'IS_ERR'
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>

security/keys/encrypted-keys/masterkey_trusted.c

@@ -16,6 +16,7 @@
 
 #include <linux/uaccess.h>
 #include <linux/module.h>
+#include <linux/err.h>
 #include <keys/trusted-type.h>
 
 /*