x86/ras: Fix build warnings in <linux/aer.h>

Fix build warning due to a missing forward declaration in
<linux/aer.h>.  We need struct pci_dev to be forward declared so we
can define pointers to it, but we don't need to pull in the whole
definition.

build log:

In file included from include/ras/ras_event.h:11:0,
                 from drivers/ras/ras.c:13:
include/linux/aer.h:42:129: warning: ‘struct pci_dev’
declared inside parameter list [enabled by default]

include/linux/aer.h:42:129: warning: its scope is only
this definition or declaration, which is probably not
what you want [enabled by default]

include/linux/aer.h:46:130: warning: ‘struct pci_dev’
declared inside parameter list [enabled by default]

include/linux/aer.h:50:136: warning: ‘struct pci_dev’
declared inside parameter list [enabled by default]

include/linux/aer.h:57:14: warning: ‘struct pci_dev’
declared inside parameter list [enabled by default]

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/53d7dea511471321bb@agluck-desk.sc.intel.com
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>