libnotmuch: export Xapian typeinfo symbols

The lack of such exporting seems to cause problems catching
exceptions, as suggested by

    http://gcc.gnu.org/wiki/Visibility

This manifested in the symbol-hiding test failing when notmuch was
compile with gcc 4.4.5. On i386, this further manifested as notmuch
new failing to run (crashing with an uncaught exception on first run).