enum: Don't directly include enum.h

To allow enum.h to depend on utils.h definitions, avoid its direct inclusion.
Instead include utils.h, which includes enum.h as well.

src/libhydra/attributes/attributes.h

@@ -24,7 +24,7 @@
 
 typedef enum configuration_attribute_type_t configuration_attribute_type_t;
 
-#include <utils/enum.h>
+#include <utils/utils.h>
 
 /**
  * Type of the attribute, as in IKEv2 RFC 3.15.1 or IKEv1 ModeConfig.