Include <algorithm> for std::sort()

Hopefully helps a Gentoo tinderbox.

Change-Id: I2e83b867113ba04a708c9fbb46c728368c4328c0
diff --git a/xmloff/source/style/impastpl.cxx b/xmloff/source/style/impastpl.cxx
index 8aca0b1..c41b5dc 100644
--- a/xmloff/source/style/impastpl.cxx
+++ b/xmloff/source/style/impastpl.cxx
@@ -17,6 +17,8 @@
 *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
 */

#include <algorithm>

#include <rtl/ustrbuf.hxx>
#include <tools/debug.hxx>
#include <xmloff/PageMasterStyleMap.hxx>