[core] workaround for AIX mmap define
[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

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4