PCI/MSI: Provide missing stubs for CONFIG_PCI_MSI=n

pci_msix_alloc_irq_at() and pci_msix_free_irq() are not declared when
CONFIG_PCI_MSI is disabled.

Users of these two calls do not yet exist but when users do appear (shown
below is an attempt to use the new API in vfio-pci) the following errors
will be encountered when compiling with CONFIG_PCI_MSI disabled:

drivers/vfio/pci/vfio_pci_intrs.c:461:4: error: implicit declaration of\
        function 'pci_msix_free_irq' is invalid in C99\
        [-Werror,-Wimplicit-function-declaration]
                           pci_msix_free_irq(pdev, msix_map);
                           ^
drivers/vfio/pci/vfio_pci_intrs.c:511:15: error: implicit declaration of\
        function 'pci_msix_alloc_irq_at' is invalid in C99\
        [-Werror,-Wimplicit-function-declaration]
                   msix_map = pci_msix_alloc_irq_at(pdev, vector, NULL);

Provide definitions for pci_msix_alloc_irq_at() and pci_msix_free_irq() in
preparation for users that need to compile when CONFIG_PCI_MSI is
disabled.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 34026364 ("PCI/MSI: Provide post-enable dynamic allocation interfaces for MSI-X")
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/158e40e1cfcfc58ae30ecb2bbfaf86e5bba7a1ef.1675978686.git.reinette.chatre@intel.com