util: Add virFileAccessibleAs to private symbols

Commit 458b7099 introduced this
function. However it was not added into libvirt_private.syms so
the compilation may not succeed on some hosts.

src/libvirt_private.syms

@@ -1094,6 +1094,7 @@ virEnumToString;
 virEventAddHandle;
 virEventRemoveHandle;
 virFileAbsPath;
+virFileAccessibleAs;
 virFileBuildPath;
 virFileExists;
 virFileFindMountPoint;