build: update gnulib submodule to latest

build: update gnulib submodule to latest

This includes a change to propagate the 4th "mount root"
field from /proc/self/mountinfo from the mountlist module,
which is needed in a subsequent commit in df.

* gl/lib/regcomp.c.diff: Regenerate against latest gnulib.
* gl/lib/regex_internal.c.diff: Likewise.
* gl/lib/regex_internal.h.diff: Likewise.
* cfg.mk: Exclude diffs from trailing whitespace check,
which is generally correct, and now needed.