Create empty.h (used to force rebuilds of version.o by the automake

Create empty.h (used to force rebuilds of version.o by the automake
makefile) as a side effect of running mkfiles.pl.

The automake docs observe that the BUILT_SOURCES list is only
automatically built by plain 'make' or 'make all' or a couple of other
targets, so the sequence './configure && make plink' from a freshly
unpacked tar file would previously fail for lack of empty.h.

If empty.h had important _content_ that needed to be built at compile
time, of course, I wouldn't be able to fix it like this; but since the
only important thing is the timestamp, I can just make sure it already
exists at the time of first build.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9288 cda61777-01e9-0310-a592-d414129be87e