Fixes compilation error reported in bug 669340
modified from upstream github issue 

Gentoo-Bug-URL: https://bugs.gentoo.org/669340
Project-Bug-URL: https://github.com/klayoutmatthias/klayout/issues/127

diff -urN klayout-9999-orig/src/db/db/dbCellMapping.cc klayout-9999/src/db/db/dbCellMapping.cc
--- klayout-9999-orig/src/db/db/dbCellMapping.cc	2018-10-26 00:57:08.027327417 +0200
+++ klayout-9999/src/db/db/dbCellMapping.cc	2018-10-26 01:00:22.757566575 +0200
@@ -75,6 +75,12 @@
 	return *this;
   }
 
+  SortedCellIndexIterator &operator--()
+  {
+    --m_n;
+    return *this;
+  }
+
   SortedCellIndexIterator &operator+=(size_t n) 
   {
     m_n += n;