If LibXML was compiled with an older ( or for that matter, newer ) version you will need to rebuild it. MinGW doesn't do anything unless you specifically invoke it, which you haven't mentioned, so I'll assume you might not have done.
From the command prompt navigate to BlitzMax\Bin and then type
bmk makemods bah.libxml
That will recompile it with the version of BlitzMax you are using, making it compatible to link against.
From the command prompt navigate to BlitzMax\Bin and then type
bmk makemods bah.libxml
That will recompile it with the version of BlitzMax you are using, making it compatible to link against.