[core] workaround for AIX mmap define

AIX might #define mmap mmap64 and this conflicts with .mmap member
of struct chunk in chunk.h

x-ref:
  ".mmap in chunk.c on AIX 5.2"
  https://redmine.lighttpd.net/boards/2/topics/7512?r=7513

src/chunk.h

@@ -2,6 +2,10 @@
 #define _CHUNK_H_
 #include "first.h"
 
+#ifdef _AIX  /*(AIX might #define mmap mmap64)*/
+#include "sys-mmap.h"
+#endif
+
 #include "buffer.h"
 #include "array.h"