sim: assume sys/stat.h always exists (via gnulib)

sim: assume sys/stat.h always exists (via gnulib)

We have many uses of sys/stat.h that are unprotected by HAVE_SYS_STAT_H,
so this is more formalizing the reality that we require this header.
Since we switched to gnulib, it guarantees that a sys/stat.h exists
for us to include, so we're doubly OK.