Add newlines to help text for dfflegalize

Add newlines to help text for dfflegalize

I think these were probably missed by accident. Spotted because GCC
spits out lots of messages like this:

passes/techmap/dfflegalize.cc:114:7: warning: zero-length gnu_printf format string [-Wformat-zero-length]
  114 |   log("");
      |       ^~

(because we tell GCC that the first argument to log() looks like a
printf control string in log.h, and a zero length such string triggers
a warning).