fix build without sfsexp

a1d139de ("lib: add sexp: prefix to Xapian (infix) query parser.",
2022-04-09) introduced sfsexp infix queries. This requires the infix
preprocessor to be built in in a way which does not require sfsexp when
notmuch is built without it.

Make the preprocessor throw a Xapian error in this case (and fix the
build).

Signed-off-by: Michael J Gruber <git@grubix.eu>