KEYS: Add forward declaration in asymmetric-parser.h

Add forward declaration for struct key_preparsed_payload so that
this header file is self-contained.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/keys/asymmetric-parser.h

@@ -10,6 +10,8 @@
 #ifndef _KEYS_ASYMMETRIC_PARSER_H
 #define _KEYS_ASYMMETRIC_PARSER_H
 
+struct key_preparsed_payload;
+
 /*
  * Key data parser.  Called during key instantiation.
  */