Fix #5181

Fixes the issue #5181, see http://hub.qgis.org/issues/5181

Compares if the selected layers in Distance Matrix tool are the same. If true, the  minimum value of nearest neighbor is set to two, avoiding compare the feature only with itself.

When the minimum nearest neighbor is set to two and the Distance Matrix tool is performed on same layer, the features will be compared with itself and with the next nearest neighbor.