Fix a minor bug in my last commit that caused the debug build to fail.

eeschema/netlist.cpp

@@ -46,6 +46,9 @@
 #include <boost/foreach.hpp>
 
 
+const SCH_SHEET_PATH BOM_LABEL::emptySheetPath;
+
+
 // Buffer to build the list of items used in netlist and erc calculations
 NETLIST_OBJECT_LIST g_NetObjectslist;
 

eeschema/netlist.h

@@ -485,11 +485,11 @@ class BOM_LABEL
     // have to store it here since the object references will be duplicated.
     SCH_SHEET_PATH m_sheetPath;  //composed of UIDs
 
-    static SCH_SHEET_PATH emptySheetPath;
+    static const SCH_SHEET_PATH emptySheetPath;
 
 public:
     BOM_LABEL( KICAD_T aType = TYPE_NOT_INIT, SCH_ITEM* aLabel = NULL,
-                  const SCH_SHEET_PATH& aSheetPath = emptySheetPath )
+               const SCH_SHEET_PATH& aSheetPath = emptySheetPath )
         : m_type( aType )
         , m_label( aLabel )
         , m_sheetPath( aSheetPath )