PCI: endpoint: pci-epf-test: Select configfs

Like some of the other endpoint modules, pci-epf-test now also uses
configfs, but is missing an indication in Kconfig:

arm-linux-gnueabi-ld: drivers/pci/endpoint/functions/pci-epf-test.o: in function `pci_epf_test_add_cfs':
pci-epf-test.c:(.text.pci_epf_test_add_cfs+0x2c): undefined reference to `config_group_init_type_name'

Select the symbol as needed.

Fixes: ffcc4850a161 ("PCI: endpoint: pci-epf-test: Allow overriding default BAR sizes")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602180706.VtXkmtqL-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://patch.msgid.link/20260211070812.4087119-1-arnd@kernel.org