PCI/PTM: Build debugfs code only if CONFIG_DEBUG_FS is enabled

PCI/PTM: Build debugfs code only if CONFIG_DEBUG_FS is enabled

Otherwise, the following build error will happen for CONFIG_DEBUG_FS=n &&
CONFIG_PCIE_PTM=y:

  drivers/pci/pcie/ptm.c:498:25: error: redefinition of 'pcie_ptm_create_debugfs'
    498 | struct pci_ptm_debugfs *pcie_ptm_create_debugfs(struct device *dev, void *pdata,
	|                         ^
  ./include/linux/pci.h:1915:2: note: previous definition is here
   1915 | *pcie_ptm_create_debugfs(struct device *dev, void *pdata,
	|  ^
  drivers/pci/pcie/ptm.c:546:6: error: redefinition of 'pcie_ptm_destroy_debugfs'
    546 | void pcie_ptm_destroy_debugfs(struct pci_ptm_debugfs *ptm_debugfs)
	|      ^
  ./include/linux/pci.h:1918:1: note: previous definition is here
   1918 | pcie_ptm_destroy_debugfs(struct pci_ptm_debugfs *ptm_debugfs) { }
	|

Fixes: 132833405e61 ("PCI: Add debugfs support for exposing PTM context")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Closes: https://lore.kernel.org/linux-pci/20250607025506.GA16607@sol
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Link: https://patch.msgid.link/20250608033305.15214-1-manivannan.sadhasivam@linaro.org