pci_regs.h: import from linux

It seems to make sense to import pci_regs.h from linux:
why maintain our own?
As a first step, move the header to standard-headers,
and add it to the update script.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

scripts/update-linux-headers.sh

@@ -31,7 +31,7 @@ fi
 cp_virtio() {
     from=$1
     to=$2
-    virtio=$(find "$from" -name '*virtio*h' -o -name "input.h")
+    virtio=$(find "$from" -name '*virtio*h' -o -name "input.h" -o -name "pci_regs.h")
     if [ "$virtio" ]; then
         rm -rf "$to"
         mkdir -p "$to"