Fix operationAsString Export (#1309)

The `operationAsString` function is used in the ADIOS1 backend, which
does symbol hiding to wrap ADIOS1's MPI mock library. Thus, we need
to export this symbol once we wrap ADIOS1.

Author: ax3l

include/openPMD/IO/IOTask.hpp

@@ -67,7 +67,7 @@ namespace internal
      * The returned strings are compile-time constants, so no worries about
      * pointer validity.
      */
-    std::string operationAsString(Operation);
+    OPENPMDAPI_EXPORT std::string operationAsString(Operation);
 } // namespace internal
 
 struct OPENPMDAPI_EXPORT AbstractParameter