Fix warning in printf format

[Philippe23]

Fix warning in printf format

In file included from fts-backend-xapian.cpp:57:
fts-backend-xapian-functions.cpp: In function 'bool fts_backend_xapian_index(xapian_fts_backend*, const char*, icu::UnicodeString*)':
fts-backend-xapian-functions.cpp:1039:95: warning: format '%ld' expects argument of type 'long int', but argument 3 has type 'int32_t' {aka 'int'} [-Wformat=]
 1039 | bose>1) i_info("FTS Xapian: fts_backend_xapian_index %s : %ld",field,data->length());
      |                                                           ~~^        ~~~~~~~~~~~~~~
      |                                                             |                    |
      |                                                             long int             int32_t {aka int}
      |                                                           %d

[grosjo]

thanks