Fix ax.c:gdb_eval_agent_expr

In C++ mode:

 src/gdb/gdbserver/ax.c: In function ‘eval_result_type gdb_eval_agent_expr(eval_agent_expr_context*, agent_expr*, ULONGEST*)’:
 src/gdb/gdbserver/ax.c:1335:11: error: invalid conversion from ‘int’ to ‘eval_result_type’ [-fpermissive]
     return 1;
	    ^

"1" as an enum eval_result_type is expr_eval_empty_expression, but
clearly this wants to return expr_eval_unhandled_opcode.

gdb/gdbserver/ChangeLog:
2015-08-21  Pedro Alves  <palves@redhat.com>

	* ax.c (gdb_eval_agent_expr): Return expr_eval_unhandled_opcode
	instead of literal 1.

gdb/gdbserver/ax.c

@@ -1332,7 +1332,7 @@ gdb_eval_agent_expr (struct eval_agent_expr_context *ctx,
 		    op);
 	  /* If ever GDB generates any of these, we don't have the
 	     option of ignoring.  */
-	  return 1;
+	  return expr_eval_unhandled_opcode;
 
 	default:
 	  ax_debug ("Agent expression op 0x%x not recognized", op);