[cppyy] Add const N to stltypes dicts

This variable is used in various stl type tests as `cppyy.gbl.N` but was always loaded through the datatypes dictionary. Since we disable rootmaps there, this variable should be added (regardless since that introduces a dependency on another tests dictionary

Author: aaronj0

bindings/pyroot/cppyy/cppyy/test/stltypes.h

@@ -12,6 +12,7 @@
 #include <utility>
 #include <vector>
 
+const int N = 5;
 
 //- basic example class
 class just_a_class {

bindings/pyroot/cppyy/cppyy/test/stltypes.xml

@@ -33,5 +33,5 @@
   <function pattern="ErrorNamespace::*" />
 
   <function name="GetMyErrorCount" />
-
+  <variable name="N" />
 </lcgdict>