Fix build with older versions of Boost

Change-Id: Ic5fa7c703821c23e704d2c58f9a04d0ac8ce8ef2
Reviewed-on: https://gerrit.libreoffice.org/69308Tested-by: Tomáš Chvátal <tchvatal@suse.cz>
Tested-by: Jenkins
Reviewed-by: Tomáš Chvátal <tchvatal@suse.cz>

vcl/unx/gtk3_kde5/gtk3_kde5_filepicker_ipc.cxx

@@ -58,7 +58,7 @@ OUString applicationDirPath()
     const auto utf8Path = applicationSystemPath.toUtf8();
     auto ret = boost::filesystem::path(utf8Path.getStr(), utf8Path.getStr() + utf8Path.getLength());
     ret.remove_filename();
-    return OUString::fromUtf8(OString(ret.c_str(), ret.size()));
+    return OUString::fromUtf8(OString(ret.c_str(), strlen(ret.c_str())));
 }
 
 OUString findPickerExecutable()