fix build with GDAL 3.13.

from https://github.com/lutraconsulting/MDAL/pull/523
see https://github.com/lutraconsulting/MDAL/issues/527

Index: mdal/frmts/mdal_gdal.cpp
--- mdal/frmts/mdal_gdal.cpp.orig
+++ mdal/frmts/mdal_gdal.cpp
@@ -175,7 +175,7 @@ double MDAL::DriverGdal::parseMetadataTime( const std:
 MDAL::DriverGdal::metadata_hash MDAL::DriverGdal::parseMetadata( GDALMajorObjectH gdalObject, const char *pszDomain /* = 0 */ )
 {
   MDAL::DriverGdal::metadata_hash meta;
-  char **GDALmetadata = nullptr;
+  CSLConstList GDALmetadata = nullptr;
   GDALmetadata = GDALGetMetadata( gdalObject, pszDomain );
 
   if ( GDALmetadata )
