[C++] add getter for schema id on the IrDecoder (#1077)

Author: green-heaven

sbe-tool/src/main/cpp/otf/IrDecoder.h

@@ -137,6 +137,10 @@ class IrDecoder
         return result;
     }
 
+    int schemaId() const {
+        return m_id;
+    }
+
 protected:
     // OS specifics
     static long long getFileSize(const char *filename)