scanner: Include flexdef.h at %top block of scan.l

scanner: Include flexdef.h at %top block of scan.l

config.h may define macros that alter the API of the standard library
funtions, and so it should be included before any other standard
header, even before the skeleton's standard header inclusion.

For example: config.h may #define _GNU_SOURCE that would expose the
reallocarray() prototype from <stdlib.h> on glibc 2.26+ systems. If we
include <stdlib.h> before config.h, reallocarray() would not be
available for use in lex file since the second include doesn't help
due to header guard.

For now our config.h might `#define malloc rpl_malloc` -- this
substitution must work before including stdlib.h, or else the compiler
will complain about missing prototypes, and may result in incorrect
code in scan.l (gcc warning: return makes pointer from integer without
a cast [-Wint-conversion]).

Fixes #247.