protolib: remove const qualifiers in ProtoSortedTreeTemplate

The methods PeekPrevItem() and PeekNextItem() in
ProtoSortedTreeTemplate were using a const qualifier, but not the
corresponding methods in ProtoSortedTree, causing a build failure with
gcc 7.x:

norm-1.5r6/protolib/include/protoTree.h: In member function 'ITEM_TYPE* ProtoSortedTreeTemplate<ITEM_TYPE>::Iterator::PeekPrevItem() const':
norm-1.5r6/protolib/include/protoTree.h:652:93: error: no matching function for call to 'ProtoSortedTreeTemplate<ITEM_TYPE>::Iterator::PeekPrevItem() const'
                     {return static_cast<ITEM_TYPE*>(ProtoSortedTree::Iterator::PeekPrevItem());}
                                                                                             ^
norm-1.5r6/protolib/include/protoTree.h:565:23: note: candidate: ProtoSortedTree::Item* ProtoSortedTree::Iterator::PeekPrevItem() <near match>
                 Item* PeekPrevItem()
                       ^~~~~~~~~~~~

Since the same methods in ProtoTreeTemplate don't have the const
qualifier, we take the simple solution of dropping such qualifiers
from ProtoSortedTreeTemplate as well, which fixes the build of norm
with gcc 7.x.

Many thanks to Romain Naour <romain.naour@gmail.com> for pointing out
the solution.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Patch has been submitted upstream and accepted, but mailing list
archive not updated.]

Index: b/protolib/include/protoTree.h
===================================================================
--- a/protolib/include/protoTree.h
+++ b/protolib/include/protoTree.h
@@ -648,12 +648,12 @@
                 
                 ITEM_TYPE* GetPrevItem()
                     {return static_cast<ITEM_TYPE*>(ProtoSortedTree::Iterator::GetPrevItem());}
-                ITEM_TYPE* PeekPrevItem() const
+                ITEM_TYPE* PeekPrevItem()
                     {return static_cast<ITEM_TYPE*>(ProtoSortedTree::Iterator::PeekPrevItem());}
                 
                 ITEM_TYPE* GetNextItem()
                     {return static_cast<ITEM_TYPE*>(ProtoSortedTree::Iterator::GetNextItem());}
-                ITEM_TYPE* PeekNextItem() const
+                ITEM_TYPE* PeekNextItem()
                     {return static_cast<ITEM_TYPE*>(ProtoSortedTree::Iterator::PeekNextItem());}
 
         };  // end class ProtoSortedTreeTemplate::Iterator