No LINGUAS file yet for pg_combinebackup

Since dc2123400, any "make" in src/bin/pg_combinebackup whines

cat: ./po/LINGUAS: No such file or directory

if you have selected --enable-nls.  This is evidently from

AVAIL_LANGUAGES := $(shell cat $(srcdir)/po/LINGUAS)

I don't particularly care to see that warning until whenever
it is that the translations first get populated.  Perhaps
we should hack up nls-global.mk to hide the warning, but
that might bite somebody someday.  I'm inclined to just
add an empty LINGUAS file as a temporary measure.
Thoughts?

            regards, tom lane