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: 1328334 ("PCI: Add debugfs support for exposing PTM context")
Reported-by: Eric Biggers <[email protected]>
Closes: https://lore.kernel.org/linux-pci/20250607025506.GA16607@sol
Signed-off-by: Manivannan Sadhasivam <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Tested-by: Eric Biggers <[email protected]>
Reviewed-by: Kuppuswamy Sathyanarayanan <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: Mani-Sadhasivam

drivers/pci/pcie/ptm.c

@@ -254,6 +254,7 @@ bool pcie_ptm_enabled(struct pci_dev *dev)
 }
 EXPORT_SYMBOL(pcie_ptm_enabled);
 
+#if IS_ENABLED(CONFIG_DEBUG_FS)
 static ssize_t context_update_write(struct file *file, const char __user *ubuf,
 			     size_t count, loff_t *ppos)
 {
@@ -552,3 +553,4 @@ void pcie_ptm_destroy_debugfs(struct pci_ptm_debugfs *ptm_debugfs)
 	debugfs_remove_recursive(ptm_debugfs->debugfs);
 }
 EXPORT_SYMBOL_GPL(pcie_ptm_destroy_debugfs);
+#endif