libdtrace: Trivial style fixes to force dt_lex.c to be regenerated

After commit 8ba333e02e ("libdtrace: Stop relying on lex
compatibility"), there have been several reports of incremental
buildworlds failing since make does not know that dt_lex.c needs to be
regenerated, and I want to avoid this when merging to stable/13.

MFC with: 8ba333e02e