Include config.h in scan.l.

Include config.h in scan.l.

This is required for things to work.  Unfortunately, %top{...} appears
to be the only bit where config.h can be put in a flex scanner, as
headers are included before the first %{...} block is output.  Thus, the
include ends up in the scan.h header file.  Nevertheless, this should
not be a problem since config.h only defines macros.