pgsql: Although the flex documentation avers that yyalloc and yyrealloc

Log Message:
-----------
Although the flex documentation avers that yyalloc and yyrealloc take
size_t arguments, the emitted scanner actually prototypes them with
type yy_size_t, which is sometimes not the same thing depending on
flex version and platform.  Easiest fix seems to be to use yy_size_t.
Per buildfarm results.

Modified Files:
--------------
    pgsql/src/backend/parser:
        scan.l (r1.155 -> r1.156)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l?r1=1.155&r2=1.156)