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/enum.c

@@ -17,6 +17,7 @@
 #include <stdio.h>
 
 #include <library.h>
+#include <utils/utils.h>
 
 #include "enum.h"