libdtrace: Stop relying on lex compatibility

It does not appear to be required, and as of commit 6b7e592c215f
("lex: Do not let input() return 0 when end-of-file is reached") it
causes input to return 0 instead of EOF when end-of-input is reached.

PR: 253440
MFC after: 3 days
Sponsored by: The FreeBSD Foundation