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/libstrongswan/utils/debug.h

@@ -26,7 +26,7 @@ typedef enum level_t level_t;
 
 #include <stdio.h>
 
-#include "utils/enum.h"
+#include <utils/utils.h>
 
 /**
  * Debug message group.