[roottest] Replace a function that doesn't exist.

Since roottest-root-io-newstl-nolib is marked WILLFAIL, it was not
noticed in #18859 that e60640c introduced a function that does not
exist.
The test still fails, but that's a different issue to be investigated.

Author: hageboeck

roottest/root/io/newstl/readNoLib.C

@@ -5,7 +5,7 @@ int readNoLib(const char *fname = "vector.root")
    dirname.ReplaceAll("/","-");
 
    TString filename = fname;
-   gSystem->PrependFilePath(dirname, filename);
+   gSystem->PrependPathName(dirname, filename);
 
    auto file0 = TFile::Open(filename);
    if (!file0)