qualified a load of size_ts with std:: namespace (all those not from Clara)

include/internal/catch_stringref.h

@@ -25,7 +25,7 @@ namespace Catch {
     class StringRef {
         friend struct StringRefTestAccess;
 
-        using size_type = size_t;
+        using size_type = std::size_t;
         
         char const* m_start;
         size_type m_size;