strigi (needed for KDE): patch to compile on stdenv-updates

This patch exports a class with STRIGI_EXPORT that wasn't exported
before.

This fixes the following build error.  I haven't tested the library, but at least it compiles now.

    [ 66%] Building CXX object libstreamanalyzer/plugins/indexers/clucenengindexer/CMakeFiles/lucene2indexer.dir/luceneindexer.cpp.o
    building libstreamanalyzer/plugins/indexers/clucenengindexer/lucene2indexer
    Linking CXX executable lucene2indexer
    ../../../lib/libstreamanalyzer.so.0.7.8: undefined reference to `Strigi::BufferedStream<char>::reset(long)'
    ../../../lib/libstreamanalyzer.so.0.7.8: undefined reference to `Strigi::BufferedStream<char>::read(char const*&, int, int)'
    collect2: error: ld returned 1 exit status
    make[2]: *** [libstreamanalyzer/plugins/indexers/clucenengindexer/lucene2indexer] Error 1
    make[1]: *** [libstreamanalyzer/plugins/indexers/clucenengindexer/CMakeFiles/lucene2indexer.dir/all] Error 2
    make: *** [all] Error 2
    builder for `/nix/store/d4lq1kfzjl68kzzk0867vydy9ds1acid-strigi-0.7.8.drv' failed with exit code 2

I am providing this patch to you under an open source license. Because I have done this work with my own personal resources, the license you receive to my code is from me and not from my employer (Facebook).