x86: Add missing pci_dev forward declaration in asm/pci.h

struct pci_dev is used in function prototypes within the header, so it must
be forward declared for asm/pci.h not to depend on xen/pci.h being included
first.

Not a functional change.

Signed-off-by: Alejandro Vallejo <agarciav@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/x86/include/asm/pci.h

@@ -13,6 +13,8 @@
                         || (id) == 0x01128086 || (id) == 0x01228086 \
                         || (id) == 0x010A8086 )
 
+struct pci_dev;
+
 struct arch_pci_dev {
     vmask_t used_vectors;
     /*