Add missing pglite/static/excluded.pglite.exports

[sriray]

src/backend/Makefile's pglite-exported-functions target does:

cat $(top_builddir)/pglite/static/excluded.pglite.exports \
$(top_builddir)/src/interfaces/libpq/exports.list \
...

but pglite/static/excluded.pglite.exports has never been committed to the repo (checked full history of that path — zero commits). build-pglite.sh calls this target directly (line ~149) with nothing upstream of it that creates the file, so a build from a clean checkout fails immediately:

$ git clone -b REL_18_3-pglite https://github.com/electric-sql/postgres-pglite
$ cd postgres-pglite
$ cat pglite/static/excluded.pglite.exports src/interfaces/libpq/exports.list
cat: pglite/static/excluded.pglite.exports: No such file or directory
cat: src/interfaces/libpq/exports.list: No such file or directory
$ echo $?
1

(the second miss there is just because this repro skips ./configure, which generates exports.list; the first one is the actual bug.)

Fix: add the missing file, empty, which is what an exclusion list with nothing excluded by default should be — the same shape as the sibling included.pglite.exports file, just with no entries.

Tested by re-running the exact cat invocation from the Makefile target against a fresh clone with the file added; it now succeeds (exit 0) instead of failing on the first argument.

No functional/behavioral change — this only unblocks the build step from erroring out before it can even start assembling the export list.