[RF] Add missing declarations for JSONInterface template specializations

In the RooFit `JSONInterface` there are some template specializations
that are defined in the translation unit but are not declared in the
header file.

This causes linker errors, because the compiler doesn't know that it has
to look for the specializations, as explained here:
https://stackoverflow.com/questions/11773960/do-template-specialisations-belong-into-the-header-or-source-file

To solve this problem, these small one-line functions are just inlined
in the header.

roofit/jsoninterface/inc/RooFit/Detail/JSONInterface.h

@@ -223,6 +223,27 @@ std::vector<T> &operator<<(std::vector<T> &v, RooFit::Detail::JSONNode const &n)
    return v;
 }
 
+template <>
+inline int JSONNode::val_t<int>() const
+{
+   return val_int();
+}
+template <>
+inline double JSONNode::val_t<double>() const
+{
+   return val_double();
+}
+template <>
+inline bool JSONNode::val_t<bool>() const
+{
+   return val_bool();
+}
+template <>
+inline std::string JSONNode::val_t<std::string>() const
+{
+   return val();
+}
+
 } // namespace Detail
 } // namespace RooFit
 

roofit/jsoninterface/src/JSONInterface.cxx

@@ -69,27 +69,6 @@ std::ostream &operator<<(std::ostream &os, JSONNode const &s)
    return os;
 }
 
-template <>
-int JSONNode::val_t<int>() const
-{
-   return val_int();
-}
-template <>
-double JSONNode::val_t<double>() const
-{
-   return val_double();
-}
-template <>
-bool JSONNode::val_t<bool>() const
-{
-   return val_bool();
-}
-template <>
-std::string JSONNode::val_t<std::string>() const
-{
-   return val();
-}
-
 std::unique_ptr<JSONTree> JSONTree::create()
 {
    return std::make_unique<Tree_t>();