ipsec-tools: Don't link against libfl

We can remove all references to yywrap by adding "%option noyywrap" statements
to each flex source file which doesn't override yywrap. After this, we no longer
need to link against libfl and so no longer get errors about undefined
references to yylex.

Also change the dependency on flex to flex-native as we no longer need libfl
compiled for the target - the lexer is now self-contained.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Joe MacDonald <joe@deserted.net>