Build System: Add missing ASTMM_LIBC to flex output.

Redirect libc allocation functions to use Asterisk functions for
main/ast_expr2f.c and res/ael/ael_lex.c.  This will resolve errors
produced by astmm.h when these files are regenerated, though other
issues still remain.

ASTERISK~27813

Change-Id: I7263e9e4217a17bde4ffaa2087a8f8aeb2a8588c