Fix the VS2015 build of unit-json_view_positions.cpp (#5957)

VS 2015 does not accept std::string::npos in a constant expression (C2131),
and the constant is only used at run time.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_positions.cpp

@@ -32,7 +32,8 @@ using nlohmann::json_editable_document;
 
 namespace
 {
-constexpr std::size_t npos = std::string::npos;
+// not constexpr: VS 2015 does not accept std::string::npos in a constant expression (C2131)
+const std::size_t npos = std::string::npos;
 
 // whether every value of a has the source positions of the value of b at the
 // same place (a and b are equal; shallow test documents only)