Move the inclusion of config.h to the top of scanner.l.

This works around _LARGE_FILES difficulties on AIX. See
http://seclists.org/nmap-dev/2012/q1/459 for example.

Original patch credit David Fifield of the Nmap project
(https://svn.nmap.org/nmap/libpcap/NMAP_MODIFICATIONS/pre-configure.patch)