Fix g++ 4.8 -Wshadow for parameter/function conflicts

Unlike the later versions, g++ 4.8 produces a -Wshadow when the name of
a parameter is the same of the name of a method, so rename the (private)
parameters to avoid this.

include/wx/generic/grid.h

@@ -1140,9 +1140,9 @@ private:
     {
     }
 
-    wxGridBlocks(iterator_impl begin, iterator_impl end) :
+    wxGridBlocks(iterator_impl ibegin, iterator_impl iend) :
-        m_begin(begin),
+        m_begin(ibegin),
-        m_end(end)
+        m_end(iend)
     {
     }