PCI/MSI: Provide missing stub for pci_msix_can_alloc_dyn()

PCI/MSI: Provide missing stub for pci_msix_can_alloc_dyn()

pci_msix_can_alloc_dyn() is not declared when CONFIG_PCI_MSI is disabled.

There is no existing user of pci_msix_can_alloc_dyn() but work is in
progress to change this. This work encounters the following error when
CONFIG_PCI_MSI is disabled:

  drivers/vfio/pci/vfio_pci_intrs.c:427:21: error: implicit declaration of function 'pci_msix_can_alloc_dyn' [-Werror=implicit-function-declaration]

Provide definition for pci_msix_can_alloc_dyn() in preparation for users
that need to compile when CONFIG_PCI_MSI is disabled.

[bhelgaas: Also reported by Arnd Bergmann <arnd@kernel.org> in
drivers/net/ethernet/mellanox/mlx5/core/pci_irq.c; added his Fixes: line]

Fixes: fb0a6a268dcd ("net/mlx5: Provide external API for allocating vectors")
Fixes: 34026364df8e ("PCI/MSI: Provide post-enable dynamic allocation interfaces for MSI-X")
Link: https://lore.kernel.org/oe-kbuild-all/202303291000.PWFqGCxH-lkp@intel.com/
Link: https://lore.kernel.org/r/310ecc4815dae4174031062f525245f0755c70e2.1680119924.git.reinette.chatre@intel.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: stable@vger.kernel.org	# v6.2+