Make pg_dumpall build with the right object files under MSVC.

This fixes a longstanding but up to now benign bug in the way pg_dumpall
was built. The bug was exposed by recent code adjustments. The Makefile
does not use $(OBJS) to build pg_dumpall, so this fix removes their source
files from the pg_dumpall object and adds in the one source file it
consequently needs.