build: regenerate .png files

We don't expect people to have tools installed to regenerate .png
from .fig by default.  However, since commit 5eb3df8b, several
.fig files were updated without regenerating the .png file, and
as a result, 'make dist' ends up regenerating those five files,
or worse, failing because of missing tools (convert from
ImageMagick).

Additionally, even if the tools are present, the generation of
.png files is nondeterministic (the resulting files contain a
timestamp), which means prior to this patch, running 'make dist'
from two checkouts will end up producing different tarball
contents (two 'make dist' runs will always produce different tar
files, since tarballs also contain timestamps; but unpacking the
tarballs and doing a recursive diff will show if the contents
are unchanged).

After this patch, the timestamps are now up-to-date, and 'make
dist' no longer has anything to do for the .png files.  This gets
us closer to the goal of two checkouts being able to produce the
same tarball.

* docs/libvirt-*.png: Regenerate.