linux: Set tst-pidfd-consts unsupported for kernels headers older than 5.10

Instead of fail trying to build the compare source file.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Matheus Castanho <msc@linux.ibm.com>
Reviewed-by: Matheus Castanho <msc@linux.ibm.com>