build-sys: fix update-potfiles script

make distcheck was broken if a user had other source files
below srcdir (e.g. in $sourcedir/mybuilddir).

Moreover autoheaders like
  libblkid/src/blkid.h
  libmount/src/libmount.h
  libsmartcols/src/libsmartcols.h
made it into POTFILES.in when srcdir = builddir and if it runs again
after make. If these headers are in Potfile.in then next out-of-tree
build from dist tarball would fail. Now we exclude (hopefully) all
of them like config.h.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>